This tree is infected with the fungal pathogen that causes pine ghost canker, which can be fatal for trees. Photo by Akif Eskalen, UC Davis

Pathogen native to U.S. but had not infected pines until recently

Fungal pathogens that cause die-back in grape, avocado, citrus, nut and other crops has found a new host and is infecting conifer trees causing pine ghost canker in urban forest areas of Southern California.

The canker can be deadly to trees.

Scientists from University of California, Davis, first spotted evidence that the pathogens had moved to pines during a routine examination of trees in Orange County. Over four years, they found that more than 30 mature pines had been infected in an area of nearly 100 acres, according to a report in the journal Plant Disease.

Akif Eskalen, a professor of Cooperative Extension in the Department of Plant Pathology at UC Davis, suspects drought and other stress conditions brought on by climate change weakened the tree species, making it more susceptible to new threats.

A cross section of a tree with ghost canker, as evidenced by discoloration and cankers with irregular, indistinct margins. Photo by Akif Eskalen, UC Davis

“We have been seeing this on pine trees for the last several years,” he said. “Our common crop pathogens are finding new hosts.”

Pine ghost canker – caused by the fungal pathogens Neofusicoccum mediterraneum and Neofusicoccum parvum – usually infects the lower part of a tree's canopy, killing branches before moving on to the trunks. This dieback in some cases can be deadly.

Points of entry

The pathogens infect a tree by entering through wounds caused by either insects such as red-haired pine bark beetles or pruning – meaning trees in managed or landscaped areas could be at risk. Another route is via tiny natural openings known as lenticels that fungi can make their way through, said Marcelo Bustamante, a Ph.D. candidate in Eskalen's lab who is first author on the paper.

Spores from the fungi can disperse and the higher the prevalence means an increased chance of transmission. Rain, irrigation water and humidity by fog can trigger the right circumstances for the spores to spread, he said.

“The detection of these pathogens in urban forests raises concerns of potential spillover events to other forest and agricultural hosts in Southern California,” Bustamante and others wrote in the report.

Dead branches can indicate a canker. Detecting the fungi is not an emergency but “people should keep an eye on their plants when they see abnormalities,” Eskalen said.

Cankers are localized areas on stems, branches and tree trunks that are usually dead, discolored and sunken. On bark, the spores can look like strings of discolored dots.

The lab has posted a brochure bout how to best manage wood canker diseases.

Tips include:

* Keep your trees healthy: Proper irrigation and maintenance will keep trees strong.

* Prune dead branches to reduce sources of infestation.

* Avoid unnecessary pruning; perform structural pruning only.

Karina Elfar, Molly Arreguin, Carissa Chiang, Samuel Wells and Karen Alarcon from the Department of Plant Pathology contributed to the paper, as did experts from Disneyland Resort Horticulture Department, State University of New York's College of Environmental Science and Forestry, UC Irvine and UC Los Angeles.

¿Está preocupado por un síntoma de enfermedad en su campo pero no sabe cuál es?

¿Necesita ayuda para diagnosticar un síntoma?

¿Por qué es importante tener una identificación adecuada de las enfermedades de las plantas?

Sin una identificación adecuada, los esfuerzos de control de enfermedades pueden ser una pérdida de tiempo y dinero si se adopta un enfoque incorrecto. La implementación de medidas de control de enfermedades que no son adecuadas para manejar el agente causante de la enfermedad podría provocar más pérdidas de plantas. Las enfermedades de las plantas son causadas por parásitos infecciosos como nematodos, hongos, oomicetos, virus y bacterias. Varios síntomas pueden ser causados por una amplia variedad de organismos (Figura 1) debido a esto, la identificación adecuada de un patógeno es clave para desarrollar una estrategia de manejo.

Figura 1. Representación esquemática de las funciones básicas de una planta a la izquierda y la interferencia con estas funciones causada por tipos comunes de patógenos vegetales que se muestran a la derecha. Fuente: Agrios, G.N. 1997. Plant Pathology (4^th ed.).

Herida vs Enfermedad

Es importante comprender la diferencia entre una herida y una enfermedad para las plantas. Una herida ocurre de repente como resultado de una fuerza externa durante un período corto de tiempo por daño mecánico, abiótico o artrópodo. La enfermedad es una desviación dañina del crecimiento normal que es continuo y normalmente progresivo. Ejemplos de heridas para una planta incluyen daños por heladas, daños por insectos o daños causados por equipos agrícolas. Mientras que un ejemplo de daño por enfermedad puede ser una planta colapsada que se ha estado marchitando lentamente debido a una enfermedad del suelo.  

El triángulo de la enfermedad

La enfermedad de las plantas es un proceso dinámico y continuo y implica interacciones entre el huésped, el patógeno y el medio ambiente a lo largo del tiempo. El huésped se refiere a la planta, el cultivo o organism que hospeda un patógeno. La enfermedad ocurre cuando las tres interacciones principals del triángulo de la enfermedad ocurren al mismo tiempo: un huésped susceptible, un patógeno virulento y un ambiente favorable para el desarrollo de la enfermedad (Figura 2).

Figura 2. El triángulo de las enfermedades de las plantas con los tres factores causales necesarios de las enfermedades colocados en las esquinas. Fuente: American Phytopathological Society.

Contáctenos para diagnóstico de enfermedades

La identificación de patógenos es difícil de diagnosticar en el campo y requiere equipos, protocolos y capacitación especializados para un diagnóstico preciso.

Si sospecha que tiene enfermedades de las plantas en su granja, comuníquese con el equipo de UCCE Pequeñas Granjas Hung Doan (Asesor de Pequeñas Granjas) o Natalie Solares (Educadora de la Comunidad ) para obtener apoyo. Para obtener más información, comuníquese con Natalie Solares en nasolare@ucanr.edu.

Qué esperar: A través de un correo electrónico, una llamada de telefono o una visita a la granja, el equipo puede evaluar la situación para identificar los próximos pasos. Según sea necesario, tomaremos muestras de plantas del área sintomática y las enviaremos al laboratorio de fitopatología de vegetales y fresas dirigido por el Dr. Alexander Putman en UC Riverside. Tras el diagnóstico, nos pondremos en contacto contigo. Es útil y importante disponer de registros detallados del historial de cultivos, aplicaciones de agua, aplicaciones de fertilizantes y eventos meteorológicos importantes que pueden haber influido en el desarrollo de enfermedades.

La autora desea agradecer a Margaret G. Lloyd (Asesora de Pequeñas Granjas de UCCE para los condados de Yolo, Sacramento, y Solano) por sus sugerencias y corrección del artículo.

Fuentes:

Plant disease management for Organic Crops. UC ANR Publication 7252

Francl, L. J. The Disease Triangle: a plant pathological paradigm revisited. American Phytological Society Teaching Notes.

Agrios, G.N. 1997. Plant Pathology (4^th ed.). Academic Press.

Are you concerned with a disease symptom in your field but do not know what it is?

Do you need assistance diagnosing a symptom?

Why is it Important to have Proper Identification of Plant Diseases?

Without proper identification, disease control efforts can be a waste of time and money if an incorrect approach is taken. Deploying disease control measures that are not suitable to manage the disease-causing agent could lead to further plant losses. Plant diseases are caused by infectious parasites such as nematodes, fungi, oomycetes, viruses, and bacteria. Various symptoms can be caused by a wide variety of organisms (Figure 1), because of this, proper identification of a pathogen is key to develop a management strategy.

Injury vs Disease

It is important to understand the difference between an injury and disease for plants. An injury occurs suddenly as a result of external force over a short period of time by mechanical, abiotic, or arthropod damage. Disease is harmful deviation from normal growth that is continuous and normally progressive. Examples of injury for a plant includes frost damage, insect damage, or damage caused by farm equipment. Whereas an example of disease damage can be a collapsed plant that has been slowly wilting due to a soil borne disease.

Figure 1. Schematic representation of basic functions of a plant on the left and interference with these functions caused by common types of plant pathogens shown on the right. Source: Agrios, G.N. 1997. Plant Pathology (4^th ed.).

The Disease Triangle

Plant disease is a dynamic and continuous process and involves interactions among the host, pathogen, and the environment over time. The host refers to the plant, the crop, or organism that harbors a pathogen. Disease occurs when the three main interactions from the disease triangle occur at the same time: a susceptible host, a virulent pathogen, and an environment favorable for disease development (Figure 2).

Figure 2. The plant disease triangle with the three necessary causal factors of disease positioned at the corners. Source: American Phytopathological Society.

Contact us for Disease Diagnosis

Pathogen identification is difficult to diagnose in the field and requires specialized equipment, protocols, and training for an accurate diagnosis.

If you suspect to have plant disease on your farm, contact the Small Farms team UCCE Hung Doan (Farm Advisor) or Natalie Solares (Community Educator Specialist) for support. For further information, please contact Natalie Solares at nasolare@ucanr.edu.

What to expect: Through an email, phone call or farm visit, the team can assess the situation to identify the next steps. As necessary, we will take plant samples from the symptomatic area, and submit them to the Vegetable and Strawberry pathology laboratory led by Dr. Alexander Putman at UC Riverside. Upon diagnosis, we will follow up with you. Detailed records of crop history, water applications, fertilizer applications, and important weather events that may have influenced the development of disease are useful and important to have available.

The author would like to thank Margaret G. Lloyd (UCCE Small Farms Advisor for Yolo, Sacramento, and Solano counties) for suggestions and proofreading the article.

Sources:

Plant disease management for Organic Crops. UC ANR Publication 7252

Francl, L. J. The Disease Triangle: a plant pathological paradigm revisited. American Phytopathological Society Teaching Notes.

Agrios, G.N. 1997. Plant Pathology (4^th ed.). Academic Press.

SUMMARY

• Two broccoli plantings immediately prior to growing the verticillium-susceptible crop is recommended for best protection
• Fresh broccoli residue has greater reduction in V. wilt than dry residue
• Field tarping following fresh residue incorporation did not increase (or decrease) efficacy
• Suppression of V. dahliae is specific to broccoli and not provided by other Brassicaceae crops.
• V. dahliae isolates from 15 host crops, including tomato, eggplant, bell pepper, lettuce, potato, watermelon and strawberry, were effectively suppressed by 5 broccoli cultivars
• The most significant reduction in V. dahliae occurs 15 days post-incorporation, and continues to decline over the season.
• More mature broccoli plants have higher levels of volatile antifungal substances
• The mechanisms of action are hypothesized to include: volatile antifungal compounds, changes in the soil microbial communities and serving as a ‘dead-end host'.
• Broccoli has been shown to reduce pathogens causing Verticillium wilt and lettuce drop, but not other soilborne pathogens such as Fusarium spp..

BACKGROUND

In 1999, several UC researchers published foundational research in a paper titled, “Evaluation of broccoli residue incorporation into field soil for Verticillium wilt control in cauliflower.” Since this publication more than 20 years ago, many studies have further investigated this concept and many coastal growers, especially organic producers, have adopted broccoli rotations as a strategy for Verticillium wilt control. Today, typical implementation of this strategy is two broccoli plantings back to back prior to the crop for which Verticillium wilt suppression is desired. While California coastal vegetable production has been the framework for much of this work, the adaptability of this practice to the Sacramento Valley is very promising for management of Verticillium wilt in warm and cool season crops[1].

Verticillium wilt is caused by the soilborne fungal pathogen Verticillium dahliae. Microsclerotia, the fungal inoculum that causes infection, dwell in the soil until root exudates stimulate germination and direct the fungal hyphae towards the root. In susceptible plants, infection occurs when hyphae enter the roots right behind the root tip, and continue growth into the water-conducting vascular tissue, the xylem. Once in the xylem, hyphal growth and sporulation can move the fungus into the upper plant tissue. Plant death triggers the fungus to a reproductive stage, prompting microsclerotia formation. When infected crop residue is incorporated into the soil, microsclerotia in the crop residue are incorporated, too. Management is particularly challenging because the pathogen host range is over 300 crops and the inoculum survive upwards of 13 years (1). To establish control of the pathogen, the key is to reduce inoculum—the number of microsclerotia, below levels damaging to susceptible crops.

Microsclerotia in crop residue

Image:Thousands of microsclerotia, small, black propagules of V. dahliae, formed on susceptible crop residue and remained intact post residue incorporation. Photo credit: M. Lloyd

BROCCOLI SUPPRESSES VERTICILLIUM WILT AND DECREASES PATHOGEN PROPAGULES

Broccoli is one of the few non-host vegetables and member of the Brassicaceae family. Bok choy, broccoli raab, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, and rapini are susceptible to V. dahliae, as are black mustard, Indian mustard, oilseed rape, and turnip. In broccoli, no infection to minor infection from V. dahliae has been observed. In the case of minor infections, the pathogen does not progress beyond the roots and microsclerotia formation in the roots is repressed. Apart from the importance of selecting a non-host as a rotation crop, the glucosinolate profile of broccoli, the secondary compounds responsible for the toxic effect, differs from other brassicaceous crops

Following broccoli residue incorporation, research out of Japan demonstrated Verticillium wilt incidence of eggplant decreased by 53 % compared to eggplant without broccoli rotation (2, 3). In California Cauliflower production, disease incidence and severity were both reduced approximately 50% following broccoli residue treatments.

Broccoli did not just decrease disease incidence, but decreased the amount of pathogen inoculum (2), showing promise for longer term management. In a California study, overall reduction in the number of propagules in V. dahliae-infested plots after two broccoli crops was approximately 94%, in contrast to the five-fold increase in the number of propagules after two cauliflower crops (4). These findings corroborate earlier studies showing reductions in the numbers of soilborne microsclerotia of V. dahliae and incidence of wilt on cauliflower that were comparable to reductions caused by chloropicrin and metham sodium treatments (5). Importantly, following broccoli rotations, microsclerotia continue to decline through-out the following cropping season and remain low during the following season. In contrast, propagules in soil fumigated with chloropicrin and metham sodium declined initially but later returned to pre-treatment levels by the end of the cropping season.

MECHANISM OF SUPPRESSION

Shetty et al. (2000) reported that the effects of broccoli in reducing microsclerotia and suppressing disease may be associated with the following mechanisms: (1) production of volatile antifungal substances such as allyl-isothiocyanate (ITC) by broccoli residue, (2) increase in antagonistic microorganisms, and (3) degradation of microsclerotia melanin by ligninase/melaninase produced by soil microorganisms in the presence of broccoli lignin. ITCs are chemically similar to methylisothiocyanate, the active agent from the chemical fumigant metam sodium. Likely associated with the ability to generate these conditions, fresh broccoli residue was shown to be more suppressive than dry residue. During tissue decomposition, the glucosinolates in crucifer crops, the characteristic sulfur-containing constituents of the members of Brassicaceae responsible for their inherent pungent odor, break down to produce sulfides, isothiocyanates, thiocyanates, and nitriles that have either fungistatic or fungicidal properties (6). In addition to release of toxic compounds and microbial activity provided by broccoli residue, the plant may be serving as a ‘decoy', ‘trap crop' or ‘dead end host', further driving population numbers down (2). As described earlier, some V. dahliae infection is observed in broccoli roots, but it does not result in microsclerotia formation. By stimulating inoculum germination and preventing fungal reproduction, the number of viable microsclerotia decrease in the soil.

GROWER IMPLEMENTATION OF RESEARCH FINDINGS

To facilitate greater adaptation of rotations with broccoli in other crops susceptible to V. dahliae, Bhat and Subbarao asked the question whether isolates of V. dahliae originating from different susceptible hosts could cause wilt on broccoli. They evaluated 15 different host isolates against multiple broccoli varieties. This included tomato, eggplant, bell pepper, lettuce, potato, watermelon and strawberry, and found that only isolates from cabbage and cauliflower were weakly pathogenic. Broccoli cultivars Baccus, Greenbelt, Parasol, Patriot, and Symphony showed resistance to Verticillium infection (3). This provides some evidence for the usefulness of this method in other cropping systems.

Implementation of broccoli rotations for Verticillium wilt management is optimized when two successive broccoli crops are grown immediately prior to desired Verticillium wilt reduction. Higher amounts of glucosinolates, specifically glucobrassicin, are found in older plants. Research has reported a complete absence of glucobrassicin in broccoli seedlings, 50% of the total in immature heads (5-10 cm diameter) and the highest levels at fully developed Packman broccoli heads (15-20 cm diameter) (7). These results suggest that glucobrassicin synthesis is active during later stages of broccoli development. Plants should be mowed and finely chopped in order to disrupt the plant cells as much as possible. The greatest reductions in microsclerotia occur at soil temperatures above 68°F, and most of this reduction occurs within 15-30 days of incorporation (8). Variation in efficacy of this method is attributed to multiple factors: fluctuation in climate and cultivation conditions, physical and chemical properties of the soil, soil microbial properties, the type of broccoli cultivar used, differences in pathogen density, and variance in the susceptibility of the following crop host. The types and amounts of glucosinolates vary with the crucifer species and determine the level of plant pathogen growth reduction.

This practice could also have other potential benefits and drawbacks. Growers in California have observed for many years that where broccoli residues from processing plants are dumped onto a field, weed populations are reduced the following year (5). Thus, rotations with broccoli may have multiple pest management benefits. However, in recent years in the Sacramento Valley, crop damage from bagrada bug has been significant. Although these outbreaks have largely occurred in fall, outbreaks have occurred in the spring in this region. Members of the Brassicaceae family are the host plants for bagrada and under favorable environmental conditions would support this pest population.

This management strategy is specific to Verticillium dahliae and is not transferrable to other soilborne pathogens such as Fusarium spp.. Because these two pathogens are common in the Sacramento Valley and above ground symptoms are similar, diagnosis is important. Contact me at any time for disease diagnostic support. All visits and sample analyses are provided free of charge.

SUGGESTED READING

Koike S, Subbarao K. 2000. Broccoli residues can control Verticillium wilt of cauliflower. Calif Agr 54(3):30-33. https://doi.org/10.3733/ca.v054n03p30.

http://calag.ucanr.edu/archive/?type=pdf&article=ca.v054n03p30

REFERENCES

1 Schnathorst, W. C. 1981. Life cycle and epidemiology of Verticillium. Pages 81-111 in: Fungal Wilt Diseases of Plants. M. E. Mace, A. A. Bell, and C. H. Beckman, eds. Aca-demic Press, New York.

2 Ikeda, K., Banno, S., Furusawa, A. et al. Crop rotation with broccoli suppresses Verticillium wilt of eggplant. J Gen Plant Pathol 81, 77–82 (2015).

3 Bhat, R. G., and Subbarao, K. V. 2001. Reaction of broccoli to isolates of Verticillium dahliae from various hosts. Plant Dis. 85:141-146.

4 Xiao, C. L., Subbarao, K. V., Schulbach, K.F., and Koike, S. T. 1998. Effects of crop rotation and irrigation on Verticillium dahliae microsclerotia in soil and wilt in cauliflower. Phytopathology 88:1046-1055.

5 Subbarao, K. V., Hubbard, J. C., and Koike,S. T. 1999. Evaluation of broccoli residue incorporation into field soil for Verticillium wilt control in cauliflower. Plant Dis.83:124-129.

6 Gamliel, A., and Stapleton, J. J. 1993. Characterization of antifungal volatile compounds evolved from solarized soil amended with cabbage residues. Phytopathology 83:899-905.

7 Kushad, M.M., B.P. Klein, M.A. Wallig, E.H. Jeffery, A.F. Brown, and A.C. Kurilich. 1999. Variation of glucosinolates in vegetable crops of Brassicaoleracea. J. Agr. Food Chem. 47:1541–1548.

8 Subbarao, K. V., and Hubbard, J. C. 1996.Interactive effects of broccoli residue and temperature on Verticillium dahliae microsclerotia in soil and on wilt in cauliflower. Phytopathology 86:1303-1310.

[1] Research has also demonstrated suppression of lettuce drop caused by Sclerotinia minor from broccoli residue. Hao J, Subbarao KV, Koike ST (2003) Effects of broccoli rotation on lettuce drop caused by Sclerotinia minor and on the population density of sclerotia in soil. Plant Dis 87:159–166.