Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Late blight of tomato is present in fields in Somerset County, Maryland. The disease can be recognized by large grey to brown lesions on the leaves. Associated leaf tissue becomes necrotic leading to the “blighted” symptom. In wet weather the lesions may have a downy appearance due to sporulation of the pathogen, Phytophthora infestans. Late blight can move quickly through a field causing large yield losses. It infects leaves, stems and fruit. Tomatoes are at particular risk now because of our rainy weather and moderate temperatures, conditions that are highly conducive to disease.
Because late blight has been confirmed on the Delmarva, growers are advised to switch to a spray program that rotates some of the following combinations:
● Forum plus a protectant, such as chlorothalonil
● Previcur Flex plus a protectant
● Ranman plus a protectant
● Tanos plus a protectant
Continue using these materials until conditions no longer favor development of late blight.
There are some OMRI (Organic Material Review Institute) approved products that list late blight as a target disease. However there is limited information on their efficacy and the information available usually indicates that they are not as efficacious as “conventional” non-organic materials. With those caveats, the OMRI approved materials include basic copper sulfate (check for formulations that state that they are OMRI approved) and Sonata.
Symptoms of tomato late blight