Be Alert for Late Blight Showing Up on Tomatoes

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

Another disease we might be seeing now and in the next week or so is late blight. The weather conditions of the past week have been favorable for its development: cool (for summer temperatures) daytime and nighttime temperatures along with ample moisture. It is important to watch for this disease and treat for it as soon as it is observed or is found in your area. Symptoms of late blight on tomato leaves are irregularly shaped water-soaked lesions that appear on young leaves at the top of the plant. Under humid conditions, these lesions become brown (Fig. 1a) and white fuzzy sporulation can be seen (Fig. 1b). Eventually the leaves shrivel and die. Brown lesions with sporulation also can occur on stems and leaf pedicels (Fig. 1c). The pathogen infects tomato fruit and causes circular greasy lesions. The fruit remains firm but can become leathery and dark brown. Best management practices for late blight control are found in the 2018 Mid-Atlantic Commercial Vegetable Production Recommendations Guide found at:

Figure 1. Late blight lesion on tomato leaf (A), sporulation of late blight causing ‘fuzzy growth’ (B) and late blight lesion and sporulation on tomato stem (C).