Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
I do not have to tell you that these frequent and heavy rains we have been having over the last 2-3 weeks have really increased the amount of foliar and, at times, soil diseases in our vegetable crops. In cucurbits foliar diseases such as Alternaria, gummy stem blight and an odd one Cercospora (Fig. 1 and 2) have been found causing moderate to severe defoliation in some fields that are heavy with fruit. The large fruit load puts a strain on the plant and when conditions are right (wet weather and warm temperatures) the fungal and bacterial diseases will flourish. Phytophthora sp also has been a problem in some cucurbit fields as this organism moves best by swimming in water and a very wet or temporarily flooded field is just what it needs to move around and infect the crop causing a ‘melt down’ of the plant.
Even when a grower has been diligent about applying their foliar fungicides and copper protectant sprays, we are still going to see plants become infected with foliar pathogens under the kinds of weather conditions we have had. One of these problem pathogens is Xanthomonas campestris pv. vesicatoria, which is the causal agent of bacterial spot in tomato (Fig. 3). However it is not that straight forward as there are at least 4 different species and four different races of this pest that can cause bacterial leaf spot. Preliminary work at North Carolina State University has shown that their bacterial leaf spot in many of their tomato fields has resistance to copper sprays. Based on what I have seen in some of our tomato fields I am sure we have similar problems. However, even if your bacterial spot is not resistant it still is going to spread and get worse in fields where it was already present after all the frequent rains that we have had. I know you have heard us in Extension say this before and repeatedly, but growers need to be sure to follow good sanitation and cultural practices in their vegetable fields, which will allow for better disease management.
Some good cultural controls include: Using pathogen-free seed and disease-free transplants –including hot water treatments that can be used to kill bacteria on and in seed. Good sanitation practices including cleaning all equipment used in diseased fields, sanitation of equipment can be done safely and effectively using a power washer and a commercial sanitizer. Keep fields free from volunteers plants, weeds, and cull piles. Avoid working in fields when bacterial diseases are present and the fields are wet. Bury or remove crop debris at the end of the season and rotate with a non-host crop for at least 2-3 years.
Figure 1. Alternaria (larger tan/brown spots) and Cercospora leaf spots (arrows) on cantaloupe leaves
Figure 2. Gummy stem blight on a watermelon leaf
Figure 3. Bacterial spot on tomato leaf