Managing Phytophthora Blight (Phytophthora capsici): Chemicals or Alternatives?
Phytophthora blight is the worse vegetable pest you can get on your farm. The Greek name Phytophthora means “Plant Destroyer” and it is very good at what it does. It attacks and kills plants in the solanaceous family (peppers, tomatoes, eggplant), cucurbit family (squash, pumpkins, melons, cucumbers), and most recently has expanded its host range to include legumes (snap and lima beans). In a wet season, it can kill 80 to 100% of the plants in an infested field.
Phytophthora capsici was once thought to be an ancient form of fungus, and is closely related to some of the organisms that cause other serious vegetable diseases, such as late blight, downy mildew, Pythium, and Rhizoctonia. However, in recent years taxonomists have thrown these disease-causing organisms right out of the fungus group all together and given them their own kingdom (Chromista) where they are known as water molds, because their life cycle is so dependant upon moisture.
The worst part about Phytophthora is that it has a resting spore (oospore) that can survive in the soil for decades waiting for you to plant a susceptible host crop. It then needs only 24 to 48 hours of saturated soil, before the oospores release zoospores, which have tails similar to a sperm and which can swim to nearby plant roots to start an infection. They also produce a third type of spore (sporangia) which can be splashed onto above ground portions of the plant to start stem, branch and fruit infections. Through a combination of swimming and splashing, the disease can quickly spread down the length of the row and sometimes to adjacent rows. It can then be easily moved around the field and farm during tillage operations, in irrigation water or on infected fruit. Once a field is infected with the disease, it is essentially infected forever! From then on, the disease will steal a little from you in dry years and more and more from you on wet years.
The question is: what to do about it? How do we control it, or if we can’t control it, how do we manage it at a level that we can live with (or most of the crop can live with)? The quick fix that most farmers think about right away is chemical control. At one time, Ridomil was fairly effective at preventing Phytophthora infections. However, many strains of the organism that exist in New England soils today are highly resistant to this chemical. However, even when Ridomil was fairly effective against Phytophthora, it often worked well enough during dry conditions, but failed to provide much control at all during wet times, when you needed it the most.
In recent years, the chemical industry has released many other products that are effective against some water molds (see late blight article), and many are also registered for Phytophthora blight control (i.e. registered for peppers). These include products such as Revus, Presidio, ProPhyt, Tanos, Reason, Acrobat, Forum, and even Kocide. However, in recent tests at the University of Michigan, 83 to 99% of inoculated ‘Red Knight’ pepper plants died after being treated with each of these products weekly from planting through harvest. Would you pay big bucks and go through all the trouble of weekly applications to save less than 17% of your crop? It should be noted that these are the results that you can expect with a susceptible (non-resistant) variety such as Red Knight. In other words, you can’t depend upon chemical control alone to stop or even slow Phytophthora down.
In the same test when the researcher treated the moderately-resistant variety ‘Paladin’ with each of these products only 5 to 26% of the plants died. Revus and Presidio provided the best results with just 5 or 6% plant mortality. The message here is that combining genetic and chemical control can provide at least a moderate level of protection for your crop.
In another study, the same researcher inoculated over 2 dozen varieties of peppers with 4 different strains of Phytophthora found in Michigan. Included in the study were most of the popular Phytophthora-resistant or tolerant peppers that are currently on the market ('Paladin', ‘Aristotle’, ‘Revolution’, ‘Declaration’, and ‘Alliance’), some susceptible varieties, and some numbered breeding lines. 'Paladin' was the only commercially-available, tolerant variety that had any plants survive (63% dead plants) inoculation with a virulent (potent) strain of the disease. A second strain that was fairly virulent killed only 6% of the 'Paladin', but 25 to 31% of the 'Revolution', 'Declaration' and 'Aristotle', and 94% of the 'Alliance'. The third strain didn’t kill any 'Paladin', 'Revolution' or 'Aristotle' plants, but killed 19% of the 'Declaration' and 'Alliance'. While the last strain failed to kill any of the 'Paladin' or 'Declaration' plants but resulted in 6 and 13% mortality for the 'Alliance' and 'Revolution'.
The message here is three fold: 1) not all strains of Phytophthora are the same, 2) resistant plants may show a different level of tolerance depending upon which strain they are exposed to, and 3) most of these tolerant varieties may not have even provided a moderate (acceptable) level of protection if exposed to the more virulent strains and treated with any of the chemicals mentioned in the previous study. In other words, since most tolerant varieties have a lower level of resistance than 'Paladin', the combination of genetic and chemical control probably would have resulted in worse results (>5% mortality) than when 'Paladin' was treated with the most effective products. Also, of all the crops susceptible to this disease, resistant varieties are only available for peppers (so far).
So where does that leave us, but searching for control options? Once again, the answer lies in integrated pest management (IPM) or bringing sanitation techniques, cultural, mechanical and physical controls into the mix.
The first step is to try to prevent the introduction of spores onto your farm or from an infested field to a clean field.
You can also help prevent a steady accumulation of spores in the soil by only planting a susceptible host crop every third year (crop rotation). Crop rotation won’t provide a clean field to plant into the way it does for other diseases (i.e. early blight), but it will help prevent facing higher and higher spore levels each year. Use resistant varieties (if available) if you have infested fields.
The only weak link that this pest has is that it requires 24 to 48 hours of soil saturation to start the disease cycle. The secret is not to let the disease get going in your fields because it is real hard to get this genie back in the bottle after it gets out and going. Water management is crucial to prevent the disease cycle from starting.
Here are some water management techniques you should employ on your farm.
You can also improve drainage with site modifications.
If the disease starts in a low wet area, rogue out infected plants, and some uninfected plants around them to prevent aerial spread. Then, break beds around the infected area to sever roots in the rows and prevent zoospores from swimming up rows.
The most extreme example of water management that I have ever seen was when a grower dug several grave-like pits within an unplanted wet hole that had no natural drainage path. He couldn’t create a trench deep or long enough to drain the low area in the middle of his field, so during heavy precipitation events, he used a mud pump and a fire hose to remove the water and prevent it from backing up into his crop. It worked! Even though that field was highly infested, he had no problem with Phytophthora that year, despite the rainy season.
In summary, water management and sanitation practices can be very effective at preventing or limiting damage from this disease. Resistant varieties can also help, depending upon the variety and disease strain present. Chemical control may help, but only if you combine it with these other control options. Finally, if you do decide to use chemical control be sure to alternate between products to slow the development of resistance.
From: CropTalk Commercial Vegetable & Fruit Crops Newsletter, Vol. 6-1, May 2010
Jude Boucher, UConn Cooperative Extension System
Vegetable Crops, IPM Program Coordinator
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