- Author: Ben Faber
It's that time of year to see some drama in avocado orchards. Once healthy-looking trees can suddenly turn brown in a weekend and all the surrounding trees still look fine. And it can be quite common in some years along the coast. The winter weather will have mild, cool even rainy days and then suddenly there's one of those 97 deg days and the tree goes down,
The entire tree or only one or several branches wilt suddenly when affected by Verticillium wilt. Leaves turn brown and die, but the dead leaves usually remain on the tree for several months. Brown to gray-brown streaks are visible in the xylem of the branches or roots when the bark is removed. Sometimes the streaking is visible in the branches, but often it is found at the base of the trunk.
Trees with Verticillium wilt often send out new, vigorous shoots within a few months after the initial wilting. If well cared for, affected trees often recover completely with no reoccurrence of the disease. However, not all trees survive an infection and disease symptoms sometimes reoccur after an apparent recovery.
The fungal pathogen Verticillium dahliae infects many hosts, including various berry and flower crops, cotton, eggplant, olive, pepper, stone fruit trees, strawberry, and tomato. Verticillium wilt is present throughout the state but is less common in avocado than root rot and canker diseases. Verticillium dahliae persists for years as microsclerotia in soil. Microsclerotia spread in infested organic matter and soil that is moved. The fungus infects through feeder roots, and then moves up in the water-conducting xylem system, restricting or preventing water movement to foliage from the roots.
No known methods are effective in curing infected trees. Trees often recover completely and display no further symptoms, even though they are still infected. After dieback ceases and new growth begins, prune off dead branches. Provide optimal irrigation and modest fertilization to promote new growth. If a tree dies from Verticillium, remove it. But give it a chance, there's a good chance it will recover.
In areas where V. dahliae is known to occur, plant Mexican rootstocks instead of the more Verticillium-susceptible Guatemalan rootstocks. Do not plant avocado on land where crops susceptible to Verticillium wilt have previously grown. Do not interplant avocado with other hosts of Verticillium, which are listed in publications such as Plants Resistant or Susceptible to Verticillium Wilt (PDF). Even if they have recovered, do not use trees infected with Verticillium wilt as a source of budwood or seed.
![avocado verticillium avocado verticillium](/blogs/blogcore/blogfiles/51899.jpg)
Help for the Home Gardener from the Contra Costa Master Gardener Help Desk
Client's Problem and Questions:
Client called (early September) and left a phone message that the her community garden colleague's tomatoes, although now 4 feet tall, set some fruit, but that the plant had suddenly started to turn yellow and appears that the plants won't surive. CCMG Help Desk responded with both a phone call and an email.
CCMG Help Desk Response:
As I mentioned in the phone message that I left for you this morning, the information in your phone message about the tomato problem your community garden colleague has experienced wasn't sufficiently detailed to allow us to diagnose the problem. You mentioned that the tomato had grown about four feet tall and had set some fruit, but that the leaves on the plant suddenly started turning yellow and it now appears that the plant won't survive.
Yellow leaves on tomatoes is associated with many different problems, including lack of nutrients, excess salts in the soil or in irrigation water, toxicity in the soil from nearby walnut trees, and many different plant diseases. You report that the plant had been growing well and producing fruit and declined very rapidly leads me to suspect that the cause may have been a vascular wilt disease.
There is nothing that can be done for plants that have Verticillium or Fusarium wilts. Plants that die should be removed and destroyed. Put the diseased plant refuse in the garbage. Don't compost it since doing so could contaminate the compost with the fungal spores. Crop rotation is of limited value as the vascular wilt fungi may survive in the soil for several years.
You can learn more about Verticillium and Fusarium wilts at these websites: http://www.ipm.ucdavis.edu/PMG/r783100911.html and http://www.ipm.ucdavis.edu/PMG/r783101011.html.
One other word of caution about something you mentioned in your phone message. You indicated that you are using free wood chip mulch from a tree service company in your community garden plot. You said that your plants are growing well and look beautiful, but you didn't mention whether you are growing vegetables or ornamental plants. We don't recommend the use of wood chip mulch in vegetable gardens. Using such mulches for vegetables leads to two different problems. One is that wood chips, if dug in, and as they haven't been fully decomposed, will rob much of the nitrogen from the soils. This depletion of a critical nutrient can affect some ornamentals. It is particularly a challenge for heavy feeders such as tomatoes. If your colleague was using wood chips as compost for her tomatoes, it's possible that one of the problems for her tomatoes was a lack of nitrogen which would also lead to yellowing of the leaves.
The other problem with using wood chip mulch in vegetable gardens is that such gardens need to be replanted after each growing season. The wood chips really get in the way when you're trying to prepare the planting beds, add compost, etc. It is not a good idea to simply work them into the soil since they will continue to decompose, using up the available nitrogen in the process.
A better mulch to use for vegetable gardens would be weed free straw or dried leaves.
Hope that this information is helpful to you and your colleague.
Contra Costa Master Gardeners Help Desk
Editor's Note: The Contra Costa Master Gardener Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523.
We can also be reached via telephone: (925) 646-6586, email: ccmg@ucanr.edu, and we are on the web at http://ccmg.ucanr.edu/
- Author: Mark Bolda
- Author: Monise Sheehan
This is simply a summary of one year of biological fungicide work in strawberries in 2012-2013 and should not be understood as a recommendation to use any of these products. This investigation is will continue into 2013-2014 and will serve to confirm and adjust the work here.
Introduction: A number of biological fungicides registered for use in strawberries have not been thoroughly tested through empirical studies to give guidance to growers on their efficacy and use.
Materials and Methods:
Table 1 below is an outline of materials tested in 2012-2013.
The field involved in the study was organically farmed and had a tested infestation of Verticillium at the average level of 25 microsclerotia per treatment replicate. Grower standard was managed as per grower practice, that is to say normal irrigation, fertility and pest management practices were applied. This grower standard did not include any sort of biological fungicide either by dip or injection through the drip tape during the season.
Plot was replicated three times and each replicate consisted of at one bed of 180 feet in length.
Application of the materials took place as indicated in the table below:
Table 1. List of treatments.
Test Material |
Application/ Use |
Dazitol |
6.25 gal/A applied 3-5 days prior to planting |
Biotam + Serenade Soil |
5 lbs/A pre-plant application 3-5 days prior to planting followed by Serenade Soil @ 4 qt/A (10 days after planting, and then Serenade Soil @ 2 qt /A applied monthly after planting. Second application of Biotam 5 lbs/A and Serenade Soil @ 4 qt/A in February. |
Serenade Soil |
Serenade Soil @ 4 qt/A (10 days after planting, and then Serenade Soil @ 2 qt /A applied monthly after planting. |
Serenade Soil |
Serenade Soil @ 4 qt/A (10 days after planting, and then Serenade Soil @ 4 qt /A applied every 60 days after planting. |
Actinovate Rate 1 |
(1) 3 oz per 100 gal root dip at planting. (2) followed by 6 oz/A pre-plant in drip tape (3) followed by 3 oz/A every 30 days in drip tape
|
Actinovate Rate 2 |
(1) 3 oz per 100 gal root dip at planting. (2) followed by 6 oz/A pre-plant in drip tape (3) followed by 6 oz/A every 30 days in drip tape
|
SoilGuard |
Apply at planting as root dip or planting furrow drench at 5 lb/ A and again through drip tape every 4-6 weeks through harvest. |
Double Nickel 55 |
Apply at planting at 1 qt /A (1/2 lb/A powder) as root dip or planting furrow drench and again through drip tape every 4-6 weeks through harvest. |
Terra Clean 5.0 |
(1) 128 fl oz/100 gal drench (2) 2 gal/A drip applied at planting (3) 1 gal/A drip applied 10 days post plant and 28 days post plant (4) 1 gal/A drip applied 60, 90 and 120 days post plant |
Terra Clean 5.0 + Serenade Soil |
(1) 128 fl oz Terra Clean /100 gal drench + Serenade Soil @ 6 qt/ A soil drench (2) 2 gal/A Terra Clean drip applied at planting followed by SS @ 3 qt /A (3) 1 gal/A Terra Clean + 3 qt /A Serenade Soil drip applied 10 days post plant and 28 days post plant (4) 1 gal/A Terra Clean + Serenade Soil @ 3 qt/ A drip applied 60, 90 and 120 days post plant |
Tainio |
(1) Spectrum @ 50 g / A + Pepzyme C @ 12.5 oz/A 2- 3 days preplant (2) Biogenesis @ 1 lb/A+ Pepzyme C @ 12.5 oz/A as plant dip (3) Pepzyme C monthly through drip tape (4) Micro 5000 @ 2.66 oz/A at 2 leaf stage foliar |
Plant dips were made by suspending the requested rate of fungicide in approximately twenty gallons of water and submerging and soaking about a half a box of strawberry transplants (about 500 plants) thoroughly and then distributing to planting crews for transplant. Note that in the case of the two Actinovate treatments, plants were held for one night to prior to transplant, ostensibly to establish the organism on the plant roots.
Drip applications were made with a portable pump injecting each fungicide. Each application normally was preceded by filling the drip tape with clear water, injecting the mix and then further pumping in clear water to make sure the fungicide had moved well out of the drip tape.
Application dates:
Preplant application – 11/3/2012
Dazitol
Terra Clean 5.0
Terraclean 5.0 + Serenade Soil
Tainio Spectrum + Pepzyme C
Biotam
Root dip + one overnight hold – 11/7/2012 (planted November 8)
Actinovate rate 1
Actinovate rate 2
Root dip and immediate planting – 11/8/2012
Double Nickel 55
Soilguard
Biogenesis + Pepzyme C
10 days post – plant – 11/20/2012
Serenade 4 qt
Serenade 2 qt
Monthly applications (12/11/2012, 1/16/2013, 2/26/2013, 3/27/2013, 4/30/2013, and 6/13/2013)
Terra Clean 5.0
Terraclean 5.0 + Serenade Soil
Tainio Spectrum + Pepzyme C
Serenade 4 qt
Serendade 2 qt
Actinovate rate 1
Actinovate rate 2
Double Nickel 55
Biotam applied 2/26/2013; Serenade 4 qt per acre applied other dates in treatment
Soilguard
Several dying plants from different areas of the test plots were sampled in July to confirm that Verticillium was the cause of plant death.
As a gauge of plant vigor from each treatment, strawberry plant diameters were measured February 13, and April 13. Measurements were in centimeters and from twenty plants in each plot.
With the beginning of fruit ripening, fruit harvest was done weekly in each treatment replicate. On each pick date, fruit from each plot was weighed and counted.
Results:
|
Plant D cm 2/13/2013 |
Plant D cm 4/13/2013 |
Yield to 5/14/2013 |
April Total Yield g/plot |
Actinovate r1 |
14.05a |
16.25a |
3894.00a |
2175.00ab |
Actinovate r2 |
13.70ab |
16.42a |
3818.33a |
2428.00a |
Biotam |
11.47b |
15.77a |
2176.67b |
1377.00b |
Dazitol |
11.70b |
17.70a |
2505.67ab |
1568.00ab |
Double Nickel 55 |
12.65ab |
15.92a |
3084.00ab |
1637.00ab |
Serenade 4 qt +2 qt |
11.57b |
16.65a |
2853.00ab |
1471.67b |
Serenade 4 qt +4 qt |
12.42ab |
16.02a |
2621.00ab |
1459.67b |
Soilguard |
11.85b |
16.78a |
2993.33ab |
1652.33ab |
Tainio |
12.93ab |
16.95a |
3454.00ab |
2075.33ab |
Terraclean |
11.50b |
16.78a |
2284.00b |
1406.67b |
Terraclean + Serenade |
12.47ab |
17.11a |
2656.33ab |
1559.67ab |
Untreated grower standard |
11.70b |
17.90a |
2145.00b |
1189.00b |
Means followed by same letter do not significantly differ (P=.05, Student-Newman-Keuls)
|
May Total Yield g/plot |
June Total Yield g/plot |
July Total Yield g/plot |
Aug Total Yield g/plot |
Total Yield g/plot |
Actinovate r1 |
3741.67a |
2791.00a |
1902.33a |
140.64a |
16820.67a |
Actinovate r2 |
3069.67a |
3237.67a |
2812.33a |
149.35a |
17201.34a |
Biotam |
2254.00a |
3251.33a |
3402.00a |
503.14a |
14468.33a |
Dazitol |
2514.67a |
3075.33a |
3049.33a |
269.16a |
14625.00a |
Double Nickel 55 |
3038.67a |
2837.67a |
2573.67a |
203.64a |
15052.67a |
Serenade 4 qt +2 qt |
3244.67a |
3287.00a |
3623.67a |
371.49a |
16887.67a |
Serenade 4 qt +4 qt |
2959.67a |
3617.67a |
3236.33a |
362.02a |
16157.00a |
Soilguard |
3212.33a |
3331.67a |
2887.00a |
359.45a |
16352.33a |
Tainio |
3409.00a |
3687.33a |
3190.33a |
170.27a |
18145.00a |
Terraclean |
2496.67a |
2997.00a |
2750.00a |
125.01a |
13681.33a |
Terraclean + Serenade |
2544.00a |
2810.00a |
2230.00a |
191.78a |
13454.33a |
Untreated grower standard |
2812.00a |
4019.33a |
3800.00a |
306.15a |
16150.33a |
Means followed by same letter do not significantly differ (P=.05, Student-Newman-Keuls)
Plant diameters measured on February 13, 2013, were significantly larger in the low rate of Actinovate than the other treatments with the exception of the high rate of Actinovate, Tainio and Double Nickel 55.
Both rates of Actinovate realized higher fruit yield than all other treatments except for the Dazitol, in cumulative fruit yield, which included six weekly harvests, up to May 14, 2013. No further differences were realized between May and August.
As noted above, this field has a very high infestation of Verticillium and all plots began to experience pronounced plant dieback in June, with some 60-80% remaining alive or declining and by end of July, nearly all plants in all replicate plots had died. There was a trend for plants which had produced larger amounts of fruit in April and May to experience lesser fruit production lesser vigor and earlier dieback as the season progressed.
Conclusion
The results of this trial are encouraging. In the early part of the season up through the middle of May, several treatments had significantly higher amounts of fruits harvested than the grower standard. Nonetheless, none of the treatments provided sufficient protection to the plants to prevent an almost complete die out of the plants by August, effectively ending the season.
- Author: Mark Bolda
- Author: Steven Koike
Marchitez de verticillium sigue siendo uno de problemas causados por patógenos del suelo de fresas cultivadas en California más potentes. Históricamente, Verticillium fue la meta principal a donde se dirigió actividades de mitigación de patógenos del suelo, en otras palabras fumigación de pre-plantar, evitación, rotación del cultivo y mejoramiento de resistencia en variedades de fresa.
Síntomas de marchitez de Verticillium: Síntomas iniciales constan de achicamiento de planta, desarrollo demorado y el amarillento de las hojas. Tal como progrese la enfermedad las hojas más maduras marchitan, resecan y tornan marrón; típicamente las hojas más jóvenes y centrales quedan verdes hasta que la planta muere y todo el follaje vuelve marrón (Foto 1). En contraste al descoloramiento del sistema vascular en marchitez de Verticillium en otros cultivos como lechuga el sistema vascular en las coronas de las plantas de fresa infectadas no es muy perceptible, y descoloramiento de la corona es apenas visible o ausente (Foto 2). Síntomas de enfermedades de fresa pueden ser acentuados si la planta infectada se sujeta a estrés tal como de extremos ambientales o infestaciones de ácaros.
El Patógeno: Marchitez de Verticilliumen fresa es causado por Verticillium dahliae. El ámbito de plantas hospederas as es amplio, aunque en los años recientes investigadores han descubierto subgrupos entre V. dahliae que tienen anfitriones preferidos y entonces son rangos aminorados. Para productores de fresa, esto quiere decir que deben de ser conscientes que aislamientos de Verticillium pueden infeccionar lechuga y alcachofa. Verticillium se nombra de la estructura de las espiras de ramas con esporos (llamado fiálides) visibles cuando se vea el hongo a través del microscopio (Foto 3).
Verticillium forma una estructura de sobrevivencia, el microsclerocio, lo cual deja el patógeno sobrevivir en condiciones no favorables y persistir entre cultivos hospederos (Foto 4). Microsclerocios son masas densas de células (melanizadas) gruesas y oscuras que forman adentro de los tejidos de la planta hospedera y sueltan una vez que los residuos del cultivo descompongan. Investigadores científicos han desarrollado métodos para medir la población de microscerocios en los suelos; tales medidas de microsclerocios por gramo (ms/g) nos comunican la potencia de Verticillium allí mismo. Un resultado de 3 ms/g nos informa de la probabilidad de que la enfermedad desarrollará en el cultivo de fresa subsecuente. Un resultado de 10 ms/g o más nos indica que es mejor no plantar fresa al menos que uno tenga planeado una fumigación. La quinta foto abajo es de un campo con un promedio de 30 ms/g de Verticillium en los últimos días de julio.
Desarrollo de la enfermedad: Microsclerocios de Verticillium germinan en el suelo cuando sean activados por exudados de la planta hospedera. Una vez penetrada la raíz, el hongo avanza en la xilema (las vías del agua en la planta), degradando las paredes de células y muy probablemente soltando elementos tóxicos además. Esta clase de colonización se llama infección sistémica. Infecciones sistémicas interfieren con la habilidad de la planta de conducir agua por lo cual las plantas infectadas marchitan en episodios de alta necesidad de agua, por ejemplo durante tiempo cálido y seco, riegos insuficientes y producción pesada de la fruta. Plantas enfermas mostrando síntomas de decaimiento pueden recuperarse en cuanto las condiciones difíciles disminuyan, pero todavía es muy probable que tales plantas no vuelven a ser tan productivas a nivel de plantas no afectadas.
El padrón y distribución de marchitez de Verticilliumen el campo puede ser extremadamente variable y no corresponden necesariamente con áreas bajas, suelos pesados o lugares no bien regadas. En contraste, estas condiciones del campo corresponderían con padrones de podrición debido a Phytophthora. En lugar de eso, muchas veces plantas afectadas por marchitez de Verticillium se encuentra distribuidas por todo el campo, quizás como plantas individuas o parchas de plantas afectadas.
El manejo de Verticillium en Fresa:
Mejoramiento de plantas para resistencia: Variedades de fresa genéticamente resistente a V. dahliae todavía no se disponen comercialmente. No obstante, resistencia tomará un papel importante en el futuro de mitigación de marchitez de Verticillium en fresa, aunque el desarrollo de variedades completamente resistentes no ha sido fácil de obtener. A según de autores de un reporte del desarrollo de resistencia a Verticillium en la fresa (California Agriculture, January/March 2010), aun una selección intensa para resistencia a este malresultó en pocos genotipos que demostraron cierta resistencia al ser inoculados con el patógeno. Sin embargo, aun estas selecciones sufrieron todavía bajas de rendimiento en presiones de enfermedad altas. Aun más, estos genotipos de alta resistencia expresaron “deficiencias substantivas para rasgos hortícolas y productivos”, lo que quiso decir que no estaban produciendo la cantidad ni la calidad de fruta que esperamos en California.
Aun así, el programa de mejoramiento de fresa de la Universidad de California ha avanzado significativamente en aumentar la resistencia a Verticillium. En la parte posterior de la década de 1980, menos de 40% de los genotipos del programa de UC tuvieron tolerancia moderada a V. dahliae, pero unos veinte años después entre 80% a 100% de estos genotipos demostraron esta resistencia.
Tratamientos del suelo: Fumigación del suelo de una mezcla de bromuro de metilo y cloropicrina usualmente es recomendada para productores convencionales de fresa, pero este plan de acción está siendo muy caro si no imposible en el ambiente regulatorio actual. Cloropicrina usada sola realiza éxito en esterilizar suelos, tanto como mezclas de 1,3- Dichloropropene y cloropicrina (Telone C-35), pero ninguno de estos se ha mostrado de ser tan eficaz como la combinación de bromuro de metilo con cloropicrina en eliminar Verticillium del suelo.
Un alternativo del tratamiento del suelo en camino de investigación y demostración es desinfestación anaeróbica del suelo (conocido por su acrónimo en ingles “ASD”). ASD funciona en inducir una condición anaeróbica en suelo enmendado con una fuente de carbón. La proliferación consecuente de microbios consumidores de oxígeno resulta mudar la ecología y diversidad microbiol del suelo y suprime los organismos patogénicos. Se sigue investigando este método.
Biofumigación es otro tratamiento del suelo que puede reducir los números de Verticillium en el suelo. Residuos de brócoli sueltan ciertas químicas que directamente reducen propágulos de Verticillium tanto como afectar la diversidad microbiol, lo que puede suprimir el patógeno. Mientras las mostazas y otras plantas crucíferas demuestran efectos similares, brócoli aparece ser uno de los mejores selecciones para biofumigación del suelo. Una rotación del cultivo incluyendo a brócoli tendrá el mismo efecto supresor ya que los floretes cosechados del brócoli no son necesarios para efectuar la biofumigación.
Saneamiento: Como es un patógeno del suelo, V. dahliae se distribuye fácilmente entre campos en lodo y suelo adhiriendo a implementos y vehículos de tal manera que productores deben prestar atención cuando trasladan implementos de campos contaminados a campos no contaminados. Residuos de cultivos de fresa además no se debe trasladar de un campo a otro, porque las plantas enfermas son infestadas de microesclerocios.
Rotación de cultivo: Rotación de cultivo es una práctica clave de MIP que aminora la amenaza de marchitez de Verticillium. Plantaciones de año tras año de fresa es una práctica de mucho riesgo si el campo tiene una historia de marchitez de Verticillium y uno no está empleando fumigantes eficaces. Campos recientemente plantados a lechuga, alcachofa y cultivos de la familia de papa (papas, berenjenas y tomates) también se debe evitar si hay una historia de Verticillium no acompañado con un buen fumigante. Malezas susceptibles a V. dahliae quizás no tienen un rol importante en el desarrollo de una enfermedad pero deben ser controlados de toda manera.
Otras alternativas: Otra alternativa experimental al control de químicas es el uso de substratos orgánicos, como los de coco o turba, los cuales se usa como medio de enraizamiento en lugar de suelo pero mantenido completamente separado del suelo a través de barreras de tela. Los substratos son introducidos a los surcos revestidos de tela. El intento de este ejercicio es crear una zona libre de patógeno encima del suelo del campo. Este método todavía está siendo fuertemente investigado.
Finalmente, hay una serie de fungicidas biológicos los cuales supuestamente excluyen hongos patogénicos como Verticillium de la superficie de la raíz, o que producen elementos tóxicos inhibitorios al crecimiento de patógenos. Estas materias todavía requieren bastante investigación y demostración de su eficacia en el campo.
Este ensayo ha sido una retrato de la biología y manejo de marchitez de Verticillium en fresa. Se menciona pesticidas para el manejo de Verticillium en el artículo. Antes de usar uno de estos productos, con su Agricultural Commissioner local y las etiquetas de los productos correspondientes para actualizarse de su uso.
Los autores agradecen a K.V. Subbarao para su ayuda en este reporte.
Referencias:
California Agriculture, Jan/Mar 2010. http://californiaagriculture.ucanr.org/landingpage.cfm?article=ca.v064n01p37&fulltext=yes
UC IPM Online, strawberry Verticillium wilt. http://www.ipm.ucdavis.edu/PMG/r734100811.html
UC IPM, Guia para el manejo de las plagas, Fresas. http://www.ipm.ucdavis.edu/PDF/PMG/pmgstrawberry_espanol.pdf
![Foto 1: Follaje marrón y decaimiento de planta causado por marchitez de Verticillium. Foto por Steven Koike, UCCE. Foto 1: Follaje marrón y decaimiento de planta causado por marchitez de Verticillium. Foto por Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17375.jpg)
![Foto 2: Corona de una planta de fresa infectada de Verticillium demostrando la carencia de descoloramiento interno. Foto por Steven Koike. Foto 2: Corona de una planta de fresa infectada de Verticillium demostrando la carencia de descoloramiento interno. Foto por Steven Koike.](/blogs/blogcore/blogfiles/17371.jpg)
![Foto 3: Vista microscópica de la estructura de soporte de esporos de V. dahliae. Foto por Steven Koike, UCCE. Foto 3: Vista microscópica de la estructura de soporte de esporos de V. dahliae. Foto por Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17372.jpg)
![Foto 4: Cultura en una placa Petri demostrando el número profuso de microsclerocios negros. Foto por Steven Koike, UCCE. Foto 4: Cultura en una placa Petri demostrando el número profuso de microsclerocios negros. Foto por Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17373.jpg)
![Foto 5: Decaimiento significativo de marchitez de Verticillium en un campo de 30 ms/g. Foto por Steven Koike. Foto 5: Decaimiento significativo de marchitez de Verticillium en un campo de 30 ms/g. Foto por Steven Koike.](/blogs/blogcore/blogfiles/17374.jpg)
- Author: Mark Bolda
- Author: Steven Koike
Verticillium wilt continues to be one of the most potentially damaging diseases caused by soilborne pathogens in strawberries grown in California. Historically Verticillium was the primary target against which soil pathogen mitigation, i.e. pre-plant soil fumigation, avoidance and crop rotation, and breeding for plant resistance, in strawberries was been directed.
Verticillium wilt symptoms: Early symptoms consist of stunting, delayed development, and the yellowing of lower leaves. As disease progresses the older leaves wilt, dry up, and become brown; typically the younger, central leaves of the plant remain green until the plant dies and all foliage turns brown (Figure 1). In contrast to Verticillium wilt of other crops such as lettuce, vascular discoloration in strawberry crowns may be subtle or absent (Figure 2). Strawberry disease symptoms can be accentuated if the infected plant is subject to stress such as from environmental extremes or mite infestations.
The Pathogen: Verticillium wilt in strawberry is caused by Verticillium dahliae. The host range of this pathogen is quite broad, though in recent years researchers have found sub-groups within V. dahliae that have preferred hosts and therefore narrower host ranges. For strawberry growers, they should be aware that the Verticillium isolates that infect lettuce and artichoke also infect strawberry. Likewise, the strawberry isolates of Verticillium can infect lettuce and artichoke. Verticillium gets its name from the whorls of spore-bearing branches (phialides) that are visible when the fungus is viewed under a light microscope (Figure 3).
Verticillium forms a survival structure, the microsclerotium, which allows the pathogen to survive unfavorable conditions and persist between host crops (Figure 4). Microsclerotia are dense masses of thick, dark (melanized) cells that form inside host tissues and are released into the soil when crop residues break down and decay. Researchers have developed methods for measuring the population of viable microsclerotia in soils; such measurements (microsclerotia per gram of soil [ms/gram]) can give growers and others an estimate of potential threat to strawberry plantings. Strawberry has a very low threshold tolerance for Verticillium. A soil test result of 3 ms/gram likely indicates that some disease may develop on the subsequent strawberry crop. With a soil test result of 10 or more ms/gram, strawberry should probably not be planted unless soil fumigation is planned. The picture below is of a field with an average of 30 ms/gram at the end of July (Figure 5).
Disease Development: Verticillium microsclerotia germinate in the soil when activated by exudates from the host plant roots. Once penetrating the plant root, the fungus grows up into the xylem (the water conducting elements of the plant), degrading the cell walls and most likely releasing toxins. This type of colonization is called a systemic infection. Systemic infections interfere with the plant’s ability to conduct water. For this reason, infected strawberry plants will wilt during times of high water demand, such as during hot and dry weather, if the plant is improperly (too dry) irrigated, or if bearing a heavy fruit load. Diseased plants that show some dieback symptoms may recover if the stress conditions subside; however, such plants are not likely to be as productive as unaffected, healthy plants.
The pattern and distribution of Verticillium wilt disease in the field can be extremely variable and does not necessarily correspond with low spots, heavy soil, or improperly irrigated areas. In contrast, such field conditions may correspond with patterns of Phytophthora crown rot. Instead, many times Verticillium wilt affected plants can be found distributed all over the field, perhaps as individual plants or as patches of affected plants.
Managing Verticillium in Strawberry:
Plant Breeding for Resistance: Strawberry plants genetically resistant to V. dahliae are not yet available commercially. However, resistance should play a big role in the future mitigation of Verticillium wilt in strawberry, though development of completely resistant cultivars has not been easily attained. In a research report (California Agriculture, January/March 2010), it is pointed out that intensive selection for Verticillium resistance resulted in a few genotypes that demonstrated a great amount of resistance when inoculated with the pathogen; however, these selections suffered some yield loss under intense disease pressure. Furthermore, these highly resistant genotypes all expressed “substantial deficiencies for horticultural or productivity traits,” meaning they weren’t producing the quantity or quality of fruit that we have come to expect in California.
Even so, the University of California strawberry breeding program has made significant advances in improving genetic resistance to Verticillium. Starting in the late 1980s, less than 40% of the genotypes used in the UC breeding program had moderate tolerance to V. dahliae; twenty years later, between 80 and 100% of the genotypes used had such tolerance.
Soil treatments: Soil fumigation with a mix of methyl bromide and chloropicrin is usually recommended for conventional growers, but this plan of action is becoming quite expensive if not impossible under new regulations and limitations. Chloropicrin used alone is successful in disinfesting soils of Verticillium, as are mixes of 1,3 – Dichloropropene and chloropicrin (Telone C-35) but generally none of these have been shown to be as effective as methyl bromide and chloropicrin used together in clearing a soil of Verticillium pathogen.
An alternative soil treatment being tested and demonstrated in several commercial fields is anaerobic soil disinfestation (ASD). ASD works by inducing an anaerobic (oxygen-less) condition in soils that are amended with a carbon source. The resulting proliferation of oxygen-consuming microbes shifts the soil ecology and microbial diversity, resulting in suppression of pathogenic organisms. Researchers are continuing to develop and fine-tune this method.
Biofumigation is another soil treatment that can reduce Verticillium numbers in the soil. Broccoli crop residues release chemicals that both directly reduce Verticillium propagules as well as affecting the soil microbial diversity, which can suppress the pathogen. While mustards and other cruciferous plants show similar effects, broccoli appears to be one of the best choices for this soil biofumigation treatment. A crop rotation that includes broccoli will have the same suppressive effect, since the harvested broccoli florets are not needed for biofumigation to take place.
Sanitation: Being a soilborne pathogen, V. dahliae is readily spread between fields in mud and dirt adhering to equipment and vehicles. Growers should therefore beware of moving contaminated equipment from infested fields into “clean” fields. Because diseased strawberry plants are infested with microsclerotia, strawberry crop residues should not be moved into other fields.
Crop rotation: Crop rotation is a key IPM practice that will help lessen the threat from Verticillium wilt. Consecutive, back-to-back plantings of strawberry is a risky practice if the field has a history of Verticillium wilt and if effective fumigants are not used. Fields which have been recently planted to lettuce, artichoke, and Solanaceous family crops (potatoes, eggplants, and tomatoes) should likewise be avoided if Verticillium wilt has occurred and if soil fumigation is not implemented. Weeds that are hosts of V. dahliae may not play a critical role in disease development but should be controlled in any case.
Other alternatives: Another experimental alternative to chemical control is the use of organic substrates, such as coconut peels (coir) or peat moss, which are used as the rooting medium in place of the soil but are kept completely separate from the soil by cloth barriers. The substrates are poured into cloth-lined furrows that are constructed into the beds. The intent of this approach is to create a pathogen-free zone above the field soil. This method is still being tested.
Finally, there are a series of commercially available biological fungicides which purport to competitively exclude pathogenic fungi such as Verticillium from the surface of the root, or which produce toxins inhibitory to pathogen growth. These materials likewise still require research and demonstration of efficacy under field conditions.
The above has been a discussion the biology and management of Verticillium wilt disease in strawberry. There are pesticides mentioned for the management of Verticillium in this article. Before using any of these products, check with your local Agricultural Commissioner’s office and consult product labels for current status of product registration, restrictions, and use information.
(The authors thank K. V. Subbarao for assistance with this report.)
References:
California Agriculture, Jan/Mar 2010. http://californiaagriculture.ucanr.org/landingpage.cfm?article=ca.v064n01p37&fulltext=yes
UC IPM Online, strawberry Verticillium wilt. http://www.ipm.ucdavis.edu/PMG/r734100811.html
UC IPM, Guia para el manejo de las plagas, Fresas. http://www.ipm.ucdavis.edu/PDF/PMG/pmgstrawberry_espanol.pdf
![Figure 1: Brown foliage and plant collapse caused by Verticillium wilt. Photo Steven Koike, UCCE. Figure 1: Brown foliage and plant collapse caused by Verticillium wilt. Photo Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17355.jpg)
![Figure 2. Crown of Verticillium-infected strawberry showing lack of internal crown discoloration.. Photo Steven Koike, UCCE. Figure 2. Crown of Verticillium-infected strawberry showing lack of internal crown discoloration.. Photo Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17344.jpg)
![Figure 3. Microscopic view of the verticillate spore bearing structure of V. dahliae. Photo Steven Koike, UCCE. Figure 3. Microscopic view of the verticillate spore bearing structure of V. dahliae. Photo Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17345.jpg)
![Figure 4. Culture petri dish showing profuse numbers of black microsclerotia. Photo Steven Koike, UCCE. Figure 4. Culture petri dish showing profuse numbers of black microsclerotia. Photo Steven Koike, UCCE.](/blogs/blogcore/blogfiles/17354.jpg)
![Figure 5. Significant dieback in a field highly infested (30 ms/gram) by V. dahliae. Photo Mark Bolda, UCCE. Figure 5. Significant dieback in a field highly infested (30 ms/gram) by V. dahliae. Photo Mark Bolda, UCCE.](/blogs/blogcore/blogfiles/17311.jpg)