Chemical/Microbial Control of Phytophthora Blight (Phytophthora capsici)

By Jude Boucher, UConn Extension

Phytophthora blight (Phyto), also known as the plant destroyer, is the toughest vegetable pest there is, because unlike other diseases where you generally start with a clean slate each year if you rotate, Phyto will survive for decades, and is a monster to manage!

Years ago the soil-applied fungicide Ridomil Gold (mefenoxam) provided pretty good control of Phyto on susceptible hosts such as solanaceous and cucurbit crops (e.g. peppers and summer squash).  However, when there is only one chemical option to control a pest, with constant exposure, it often doesn’t take long for resistance to become a problem.  Today most of the Phyto strains in the Northeast are highly resistant to Ridomil.

Now, recent research from Michigan, and other states, is showing that the old approach of applying some of the newer fungicides directly to the soil as transplant drenches, or as soil drenches at or after planting, or through trickle irrigations systems, can substantially reduce the number of plants that die from the crown rot phase of the disease, leading to higher overall yields.  When foliar and fruit infections are also a problem, follow-up foliar applications have proven to be effective.  Researchers recommend making foliar applications with three nozzles per row, with the outer nozzles angled in at 45 degrees, and with 50 g.p.a. of water so that the product covers and runs down the stem to protect the plant crown and roots.

I can’t call these preventative soil and foliar applications IPM, because one of the principles of IPM is that you never make preventative pesticide applications (applications are made only after detecting the pest and reaching an action threshold).  However, the scientists conducting this research, and most of the fungicide labels, still insist that you need to combine the use of these chemical controls with the use of cultural controls and (when possible) resistant varieties as part of an IPM system.

Cultural controls include strategies such as avoiding planting susceptible crops in infested soil, limiting the spread of the pathogen to clean fields, and any number of water management techniques that reduce soil saturation.  Some of these include: avoiding planting in low wet holes or areas that will flood sometime during the season; using raised-beds to reduce soil saturation; aligning the beds with the natural slope of the field to allow water to run out from between beds; breaking the beds at low points to allow water to leave the field through its natural course; sub-soiling or zone-tilling to break plow pans and improve drainage both before building beds and between beds after they are built.  Also, you should rogue out sick and dying plants early in the disease cycle so that the pathogen doesn’t spread down the rows or to adjacent rows.

Some of the newer fungicides that show varying degrees of promise as soil treatments or follow-up foliar treatments include:  Presidio (fluopicolide, resistance group 43), Zampro (ametoctradin + dimethomorph, groups 45 & 40), Revus (mandipropamid, group 40) and Ranman (cyzofamid, group 21).  Presidio and Ranman have supplemental labels that allow drench or trickle irrigation applications, while Zampro allows these types of soil applications on the main label, but Revus can only be applied as a foliar treatment.  Supplemental labels are available on the CDMS web site (www.cdms.com) and must be in your possession when soil applications are made.

They have even tested some experimental chemicals (i.e. oxathiapiprolin and others) from different companies that provided 100% control in field trials when soil applied.  Time will tell if these new chemicals receive registrations.  If they do get registered soon, they will help provide options to alternate with to slow resistance so that we don’t repeat what happened to Ridomil.  Labels on all these products limit their use to 3 to 6 applications per season, require alternating between products from different resistance groups, and call for tank mixing with another product or a surfactant to help combat resistance (see table).  These label restrictions, while extremely important to prevent resistance, make it challenging and expensive to design a spray program that will protect your crop throughout the season.  Here are some effective ways to use these products to fight Phyto.

Of the registered products, soil applications at planting (drenches) of Presidio, followed by foliar sprays of Revus + copper, alternated with Presidio + copper, on a 7 to 10 day interval seems to be the most effective.  In 2012 and 2013 trials in MI, this or similar spray programs completely eliminated, or dramatically reduced, plant death on peppers and produced the highest yields in the trial.  Similar results were achieved on summer squash.  When they tried the same type foliar spray programs without the Presidio soil drench at planting they didn’t work nearly as well.

Transplant drenches of Zampro or Ranman, followed by foliar programs that alternated between products, produced intermediate results in the MI studies.  So, if you wanted to use an effective all-soil-applied program, you could start with Presidio at planting, and then alternate every 7-1O days between Zampro, Ranman and Presidio.  The Ranman cannot be applied through a drip system, so it could be put on as a drench with a backpack or boom sprayer.  You could also use these same three products as foliar applications after an initial soil application of Presidio, but the foliar Presidio must be applied with another product for resistance management and the Ranman and Zampro must be applied with a silicon or non-ionic surfactant.   I think you can see how this program can get expensive.  However, don’t think about just using a single drench at planting, because that didn’t work any better than the untreated control plots in these studies.

What about Microbial products?  In one study with summer squash drench applications of either Presidio, BioTam (Trichoderma asperellum and gamsii), Serenade Soil (Bacillus subtilis), or Actinovate AG (Streptomyses lydicus), applied to the soil at planting and then every 14 days, the three microbial products reduced plant death by about half compared with the untreated control plots and improved yields by 3 to 4 fold.  Presidio again performed best by reducing plant death by about 85% and improving yields by about 5.5 fold (Note: this was for experimental purposes, the label does not allow more than 2 sequential applications).  When Presidio was alternated with any one of the Microbial products every 14 days, they did about as well as the Presidio alone, but with the added advantage of having a resistance management component to the program and less overall chemical usage.

Most of these studies were conducted by Dr. Mary Hausbeck and her students Rachel Naegele and Charles Krasnow at Michigan State University.   Thanks to Meg McGrath for sharing the results of several studies from different researchers.

 

Product	resistance group	Applications allowed	Soil or Foliar	Mix required	Days to harvest	Registered host crops	Plant back restriction
Presidio	43	4	S/F	Soil – no Foliar -yes	2	Cucurbits, fruiting	0-18 months
Ranman	21	6	S/F	Soil – no Foliar–surf	0	Cucurbits, fruiting	30 days
Revus	40	4	F	Foliar – copper/surf	0-1	Cucurbits, fruiting	30 days
Zampro	40 & 45	3	S/F	Soil – yes Foliar-surf	0-4	Cucurbits, fruiting	0-12 months