It has been a very interesting year for field diseases, and the excessively rainy summer has been very conducive to the development and spread of fungal-like organisms. These include members of the genus Phytophthora. Heavy, persistent rains have allowed these pathogens to cause symptoms on foliage, which is a very rare occurrence. Consequently, we were recently informed that the presumed late blight samples sent from Maryland and Delaware to Cornell for typing, are in fact NOT late blight. The lab in New York confirmed the new diagnoses with ITS sequencing. The sample from tomato in Montgomery Co., MD was the related pathogen Phytophthora capsici. The sample from potato in Kent Co., DE was the related pathogen Phytophthora nicotianae. Both of these pathogens are also important and damaging diseases.
Phytophthora capsisi causes crown and fruit rot on tomato. It is uncommon for P. capsici to cause lesions on tomato leaves, here in Maryland. More typically we see crown and fruit symptoms. To manage fruit rot, apply mefenoxam (Ridomil Gold™ or Ultra Flourish™) as a soil surface application under the vines 4 to 8 weeks before harvest. Another approach is to apply mefenoxam + chlorothalonil, Ridomil Gold Copper™, Gavel™ or Tanos™, when the crown fruit are one-third their final size. Repeat at 14 day intervals up to a total of 3 times.
Likewise it is unusual to see P. nicotianae on potato leaves. Leak is usually associated with tubers that have been bruised during harvest in hot weather. Management is to apply fungicides in a 6 – 8inch band directly over the seed piece during planting. Fungicides which are effective are Platinum Ridomil Gold ™, Presidio™, Ridomil Gold™, Ultra Flourish™, or Ranman™. An alternative method is to apply one of the following at flowering and 14 days later (in an high gallonage as possible, minimum of 5 gpa aerial application); Ridomil Gold Bravo™, Ridomil Gold Copper™, or Ridomil Gold MZ™. See the Commercial Vegetable Recommendation Guide for more information.
The wet weather has brought out the Phytophthora species on other crops as well. In addition to P. capsici on tomato foliage, we have seen lesions on watermelon leaves. These lesions appeared similar to the (more typical) lesions on pumpkin leaves.
Phytophthora nicotianae symptoms on potato