- Author: Help Desk Team
Do your tomato plants suffer from lack of vigor, yellowing leaves, and poor fruit production? These symptoms are associated with many different problems, including lack of soil nutrients, excess salts in the soil or irrigation water, poor drainage or waterlogged soil, or a variety of plant diseases.
Trying to figure out what has gone wrong in your garden may not sound like much fun, but it is a necessary and important part of gardening. The more information we have, the easier it is to diagnose the problem when our hopes for a bountiful garden turn to disappointment. A gardener may change how they care for ailing plants by applying more or less water, fertilizing, or even spraying with pesticides or fungicides. Often nothing they do makes a difference, the plants continue to fail, and many eventually wither and die.
If this describes your experience growing tomatoes, you may find that a wilt disease could be the culprit. Armed with the information in this article, you can determine whether a wilt disease is a problem in your garden.
What is a wilt disease?
Verticillium wilt and Fusarium wilt, also called vascular wilt diseases, are soil-borne fungal diseases that infect susceptible plants through the roots, growing into and plugging the water-conducting (vascular) tissues. This causes the plant to be unable to take up sufficient water needed for healthy growth. Verticillium and Fusarium cause similar symptoms in their hosts.
Once these pathogens are in the soil, they are difficult to manage as they can survive in the soil for many years and possibly decades without living host plants. There is nothing that can be done for plants that are already infected with either Verticillium or Fusarium wilt. There are no fungicides available for application to the plants that can be used to control these diseases.
Verticillium wilt is widespread and very destructive and is common in our area. It is estimated that over 300 plant species throughout the world are susceptible. The pathogen responsible for the disease in tomato plants is Verticillium dahliae. This fungus favors moist soil and relatively cool (55–75° F) soil temperatures.
Early symptoms include a progressive yellowing of older leaves and wilting of shoot tips at the top of the plant during the warm part of the day.
Later, leaf margins curl upward, and leaves often drop off. Any fruit produced is usually small. The wilting becomes progressively worse, and although plants may not die, their condition can be so poor that it is not worth the effort to try to keep them going. Verticillium wilt will also infect several other common garden plants.
The list is a long one, and includes many garden vegetables as well as shrubs, trees and ornamentals. Below are links to lists of plants that are susceptible to Verticillium and Fusarium wilt.
Fusarium wilt presents with symptoms that, while not identical, are quite similar to Verticillium wilt. Although it is not as widespread as Verticillium, Fusarium is also a common disease in tomatoes. It differs from Verticillium wilt in that the Fusarium oxysporum pathogen has several forms which are host-specific. This means that the specific Fusarium fungi, Fusarium oxysporum f.sp. lycopersici, only causes disease in tomato plants.
Unlike Verticillium wilt which has a very wide host range, the strain of Fusarium that infects tomatoes will not cause disease in other plant species. It can infect many different non-tomato crops, including melons, pepper, and sunflowers without causing any symptoms, and will persist in both tomato and non-tomato plant residue in soil. Infection and disease development in Fusarium wilt are favored by warm soil temperatures (80° F) and low soil moisture.
The symptoms of this disease are bright yellow leaves that commonly appear first on just one side or one branch of a plant. The leaf symptoms will also often appear as yellowing on just one side of a leaf, delineated by the central vein. These symptoms are distinctive and can be very helpful in making a diagnosis. The foliage yellows, wilts, then turns brown and dies. Older leaves are affected first, followed by death of the entire plant.
Disease confirmation
To confirm the diagnosis of a wilt disease, cut open one of the lower stems of the affected plant. If the vascular tissue inside the stem has turned brown or darkened (in contrast to the white or pale green color inside a healthy stem), it is a good indication that the plant has one or the other of these fungal diseases.
If you confirm the presence of the disease, you should remove the infected plant (including as much of the roots and plant debris as possible) and dispose of it in the garbage or your green waste bin. Don't compost it yourself since home compost piles rarely reach and maintain temperatures hot enough to kill the disease pathogens.
How did this disease get into my garden?
Both Fusarium and Verticillium fungi can be introduced on infected transplants, seeds, and tubers, or spread on equipment contaminated with infected soil. Wood chip mulch that comes from an infected tree can also spread the Verticillium fungi from one area to another. Because it can also survive in the digestive tract of horses and other animals who have consumed infected plant material, animal manures can sometimes be another source.
What steps can I take to keep these diseases from spreading throughout my garden?
- Do not transplant plants from one area to another. Moving soil, even a very small amount, can spread the disease.
- The fungi can also be transported on equipment such as shovels, trowels, etc. Thoroughly clean and disinfect tools used in the infected garden bed before using them in other areas of your yard or garden. First wash or brush off any soil or plant debris, then disinfect your tools with alcohol. You can either wipe or dip them in a solution that is 70–100% alcohol. Isopropyl alcohol is widely available and can easily be found in many stores.
- More on products used to sterilize garden tools, and their pros and cons: https://gardeningsolutions.ifas.ufl.edu/care/tools-and-equipment/disinfecting-tools.html
I suspect that I may have a wilt disease in my garden. Will I still be able to grow tomatoes and other vegetables?
Choosing disease resistant tomato varieties is a way for gardeners to prevent the losses due to wilt diseases. Many disease-resistant hybrid tomato plant varieties have been developed and are available to the home gardener. They can be found at our yearly Great Tomato Plant Sale, in seed catalogs, and at local garden centers. The letters V and F following the variety name in seed catalogs, on seed packets, or plant labels denote varieties that are resistant to Verticillium and Fusarium wilts.
It is unfortunate that nearly all heirloom tomato varieties are susceptible to wilt disease. Heirloom tomato varieties would be better grown in containers with fresh potting mix or in a garden area that has not developed the pathogens (for example, an area in which you have never previously grown tomatoes or other crops that are vulnerable to the wilt diseases).
In addition to tomatoes, several vegetable plants including peppers, eggplants, potatoes, squash, and melons can suffer from Verticillium wilt. The Fusarium fungus specific to tomatoes will only affect tomatoes and will not cause disease in other vegetables. If you plan to grow other vegetables in the same garden bed where a tomato plant suspected of having Verticillium wilt has grown. Check the links below or do an internet search to determine whether the variety you want to grow is susceptible to the wilt diseases. For example, you can search “is broccoli susceptible to Verticillium wilt?”
Soil solarization is another method that can be used to reduce the amount of these pathogens in the soil. It involves heating the soil by covering it with clear plastic for four to six weeks during the summer. It is most effective when day length at its longest, and temperatures are high.
Links to additional information:
• Soil solarization: https://ipm.ucanr.edu/PMG/PESTNOTES/pn74145.html
• Verticillium wilt: http://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/tomvertwilt.html
• Fusarium wilt: https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/fusariumwlt.html
• Plants resistant or susceptible to Verticillium wilt: https://depts.washington.edu/hortlib/resources/ucdavis_verticillium.pdf
• Plants susceptible to Fusarium wilt: https://extension.usu.edu/pests/research/fusarium-verticillium-wilts
To find great gardening info, a list of previous blog postings, and information on our Great Tomato Plant Sale go to our website: https://ccmg.ucanr.edu/
Help Desk of the UC Master Gardeners of Contra Costa County (SMH)
By Susanne von Rosenberg, UC Master Gardener of Napa County
Some gardening advice seems so sensible or obvious, or has been repeated so often, that we “know” it to be true. As gardeners, we need to be careful about falling prey to myths. In this column, we'll explore some widespread gardening myths, although perhaps I should call them confusions.
We are usually told that wilting leaves indicate that a plant needs water. This is often the case, and you can check to see if your soil is dry. But there are other reasons that a plant's leaves could be wilting.
If the soil is waterlogged, the oxygen that roots need to grow is replaced with water. This happens often to plants in containers without drainage. Plants may also become waterlogged in compacted soil that doesn't drain well. When roots lack oxygen, they can't supply water to the rest of the plant. The leaves continue to photosynthesize and to use water. By forcing roots to shut down, waterlogged soil leads to wilted leaves.
Fungal or bacterial disease can also cause leaf wilt. Fusarium and verticillium are common in local soil. These fungi block the plant's water transport structures, causing leaves to wilt even when there is plenty of soil moisture. Often the entire plant dies.
Keeping your soil too moist can promote fungal disease. Another culprit for wilting leaves is animal damage to the roots. Gophers, nematodes and other insects feed directly on roots, and other animals may create tunnels that drastically reduce root contact with surrounding soils.
Another possible cause of wilting is overfertilizing or excess salt in the soil. Always follow the directions on any packaged fertilizer, and be aware that steer manure is relatively high in salt.
You may have heard that bark chips are a better mulch for your perennial woody plants than tree chips. Some say that tree chips could contain disease-causing organisms or compounds that inhibit the growth of other trees.
Neither of these concerns has been documented as a significant issue. Conversely, bark chips, while a more attractive mulch than tree chips, have poor water retention. Tree bark is designed to prevent water loss. It contains a waxy compound, suberin, that repels water.
Tree chips, on the other hand, consist mostly of inner wood, which has the capacity to absorb and hold moisture. Depending on where the logs were stored before the bark was collected and chipped, bark chips can also contain weed seeds. Tree chips are more environmentally friendly, too, because they are local. By using them, you are reducing the amount of material that needs to be processed in a composting facility.
While we're talking about mulch, let's explode the myth that there is any permanent solution for weed control other than paving over your yard. Landscape fabric, which differs from black plastic mulch in having small holes for air and water exchange, is often touted as a permanent solution. It is used in commercial vegetable and ornamental plant production, where it is successful at keeping weeds out. It can also be easily removed and replaced each year.
However, landscape fabric is not good for permanent weed control around perennials. If left exposed, the fabric can degrade in a year. If you cover it with mulch or allow plant debris to accumulate on top of it, weeds will colonize the plant material or mulch and grow through the fabric, making it difficult to remove.
You can cover the fabric with gravel, but you'll still need to keep removing the weeds that manage to colonize gravel-covered fabric. Your plants may also grow into the fabric, so when you remove the fabric, you damage their roots.
As the fabric degrades, you'll have a clean-up and aesthetic headache, with lots of shredded bits. (I know this from personal experience.) Gophers can exert so much pressure building mounds under the fabric that the fabric rips. If it doesn't rip, you end up with exposed landscape-fabric-covered mounds.
Unfortunately, there is no magic bullet for weed control. You can reduce them, but you still need to remain vigilant no matter what method you use. Organic mulches are best as they improve soil texture and health and can be replenished as needed. Couple them with targeted watering.
If you want to learn more about garden myths, Dr. Linda Chalker-Scott, an extension horticulturist and associate professor at Washington State University, has a wonderful website that discusses all sorts of common Horticultural Myths.
Food Growing Forum: Napa CountyMaster Gardeners will present a discussion of “Perennial Vegetables, Garlic and Alliums” on Sunday, October 10, from 3 p.m. to 4 p.m., via Zoom. Register here to receive the Zoom link.
Free Guided Tree Walk: Join Master Gardeners of Napa County for a tree walk in Fuller Park in Napa on Tuesday, October 12, from 10 a.m. to noon. Limited to 12 people per walk. COVID safety protocols will be followed. You will be asked health questions and asked to sign in. Face masks and social distancing are required. Register here.
Got Garden Questions? Contact our Help Desk. The team is working remotely so please submit your questions through our diagnosis form, sending any photos to mastergardeners@countyofnapa.org or leave a detailed message at 707- 253-4143. A Master Gardener will get back to you by phone or email. For more information visit http://napamg.ucanr.edu or find us on Facebook or Instagram, UC Master Gardeners of Napa County.
![Not all wilted leaves are caused by lack of water. (gardeningknowhow.com) Not all wilted leaves are caused by lack of water. (gardeningknowhow.com)](/blogs/blogcore/blogfiles/86383.jpg)
![Waterlogged soil could be a cause for wilted leaves. (greenmylife.in) Waterlogged soil could be a cause for wilted leaves. (greenmylife.in)](/blogs/blogcore/blogfiles/86373.png)
![Fusarium wilt in strawberry. Known to affect tomatoes, too. (ucanr.edu) Fusarium wilt in strawberry. Known to affect tomatoes, too. (ucanr.edu)](/blogs/blogcore/blogfiles/86374.jpg)
![Verticillium wilt on tomatoes. Caused by a soil fungus. (ucanr.edu) Verticillium wilt on tomatoes. Caused by a soil fungus. (ucanr.edu)](/blogs/blogcore/blogfiles/86375.jpg)
![Vertebrate pests damage roots, leading to loss of water to leaves. (ucanr.edu) Vertebrate pests damage roots, leading to loss of water to leaves. (ucanr.edu)](/blogs/blogcore/blogfiles/86377.jpg)
![Using tree chips instead of bark chips conserves water. (sonorantreesvc.com) Using tree chips instead of bark chips conserves water. (sonorantreesvc.com)](/blogs/blogcore/blogfiles/86379.jpg)
![Read the label. Follow directions exactly. (bgi-usa.com) Read the label. Follow directions exactly. (bgi-usa.com)](/blogs/blogcore/blogfiles/86378.png)
![Woven landscape fabric is water and air permeable. (flowerarmor.com) Woven landscape fabric is water and air permeable. (flowerarmor.com)](/blogs/blogcore/blogfiles/86380.jpg)
![Weeds -do- grow through landscape fabric, and mulch. You might have fewer weeds, but they'll still need to be removed! (homeguides.sfgate.com) Weeds -do- grow through landscape fabric, and mulch. You might have fewer weeds, but they'll still need to be removed! (homeguides.sfgate.com)](/blogs/blogcore/blogfiles/86381.jpg)
![Blog, Linda Chalker-Scott Horticultural Myths (pinterest.com) Blog, Linda Chalker-Scott Horticultural Myths (pinterest.com)](/blogs/blogcore/blogfiles/86382.png)
- Author: Margaret Gullette Lloyd
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.
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.
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- Author: MaryJo Smith
In the garden, Verticillium Wilt (VW) can affect potatoes, peppers, tomatoes, eggplant, some cole crops, tomatoes, lettuce, strawberries, cucurbits, artichokes, avocados, peaches, nectarines, caneberries, and apricots. It can also affect landscape plants and flowers. In fact, there are over 400 plant species that are affected by the Verticillium wilt disease (VW). Given the wide range of hosts that succumbs to VW, it is important to prevent, or at least manage, the disease.
The fungus persists in soil for long periods. Although infection is favored by cool weather, because the fungus interferes with water transport in the stems, crop damage is most severe during periods of hot weather when plants are stressed for water. This is one of those diseases that can “crop” up on you (har har snort, pun intended), but with some forethought, can be avoided or at least minimized. If you suspect that your soil is infected with VW, the best way to know for sure is to have your soil tested by a plant & soil diagnostic laboratory.
Use Resistant Cultivars. If you don't have a problem with VW, then a great way to avoid it is to try to select resistant cultivars when choosing your starts. Some cultivars are more tolerant than others. Practice proper cultural practices in order to avoid stressing plants. For example, if growing artichokes, don't take crowns to be used for propagation from where the disease has occurred (plant only pathogen-free plants). Likewise, don't plant annual artichokes in an area with a history of V. dahliae. All annual artichoke varieties are more susceptible to VW than the perennial Green Globe variety so consider planting only perennial artichokes.
Crop Rotation. Another way to manage VW is to rotate your crops. Don't plant the same crop/similarly susceptible crop in the same area/bed in consecutive years. Ideally, rotate out susceptible crops for 2 to 3 years. For example, rotate the tomatoes and peppers with non-host crops such as beans, corn or broccoli. Cereals, grasses, and legumes are good rotation crops.
Biofumigation. Planting broccoli, a non-host of VW, may also help reduce pathogen levels through a process called biofumigation: decaying broccoli residue, when disced into the soil, either gives off natural chemicals that can kill VW or alters the soil microflora so that VW survival is reduced. You can also use cauliflower too, but only in winter or early spring.
Soil solarization. In warmer areas of the state, solarization is an effective control of soilborne pathogens and weeds. Solarization is carried out after the beds are formed and when the weather conditions are ideal (30-45 days of hot weather that promotes soil temperatures of at least 122°F). The effectiveness of solarization can be increased by solarizing after incorporating the residue of a cruciferous crop, in particular broccoli or mustards, into the soil.
(all information courtesy of UC Davis ANR, Integrated Pest Management Program) For more information, see the links below:
- Author: Margaret Lloyd
- Author: Tom Gordon
Verticillium dahliae is the cause of Verticillium wilt, a disease that affects strawberries and many other plant species, including annual vegetables, fruit trees, nut trees and fiber crops, as well as weeds and native plants in California. Once a susceptible plant is infected, V. dahliae can produce large numbers of survival structures (called microsclerotia) (Vallad and Subbarao, 2008), which can survive in the soil for more than 14 years (Wilhelm, 1955). In addition, V. dahliae can colonize the roots of many crops that are not susceptible to disease and may show no symptoms (Lloyd and Gordon, 2011). This provides another means by which the fungus can produce survival structures. Verticillium dahliae is easily spread between fields with soil on farming equipment. It can also be introduced with seed (Wu and Subbarao, 2014) or infected transplants. Owing to the ease of dispersal, a wide host range and production of long-lived survival structures, V. dahliae is resident in agricultural soils throughout the state. Whether or not Verticillium wilt occurs in a particular situation is determined by the susceptibility of the crop variety being grown and the abundance of inoculum in soil. Consequently, the absence of disease does not mean the pathogen is not present. In fact, it is likely that V. dahliae occurs at some level in most fields in coastal California. Good management practices aim to keep the inoculum level below a damaging threshold (Gordon and Subbarao, 2007).
Many growers use compost amendments to improve the physical and chemical properties of soil. Compost can also enhance the activity of microorganisms that are inhibitory to plant pathogenic fungi (Mazzola, 2004). However, compost made from crop residue may include plants that were infected with V. dahliae, and because microsclerotia can survive in animal guts (Markakis, 2014), manure may be contaminated as well. Proper composting is required to ensure that no viable microsclerotia remain in the final product. The California Composting Council and CalRecycle have established regulations that require materials reach 55 °C (131°F) or higher for 15 days or longer with a minimum of five turnings of the windrow during this time (Cal Recycle, Title 14, Chapter 3.1), which should be sufficient to kill V. dahliae microsclerotia in manure or crop residue (Baker, 1957). All state permitted composting facilities are visited monthly by the local enforcement authority (LEA) to ensure compliance with these regulations. Consequently, if your supplier is operating under a state permit, the compost you purchase is unlikely to be a source of inoculum of V. dahliae, and it would not be advisable to forego the benefits of compost amendments out of concern that this will increase the risk of damage from Verticillium wilt.
REFERENCES
Cal Recycle. Title 14, Chapter 3.1. Composting Operations Regulatory Requirements, Article 5. Composting Operation and Facility Siting and Design Standards, Section 17868. 3. Pathogen Reduction. http://www.calrecycle.ca.gov/Laws/Regulations/title14/ch31a5.htm
Baker, KF and Roistacher, CN. 1957. Heat treatment of soil. In K.F. Baker [ed]. The U.C. system for producing healthy container-grown plants. Calif. Agr. Exp. Sta. Manual 23:123-137.
Gordon, TR and Subbarao, KV. 2007. Production Guidelines: Verticillium wilt of Strawberry. California Strawberry Commission, Issue 1: 1-4.
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