Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
A root disease commonly associated with potato has turned up on tomato in high tunnels. This disease goes by the delightful name of black dot root rot. The causal agent Colletotrichum coccodes also causes anthracnose fruit rot on tomato (sunken, water-soaked, circular lesions).
C. coccodes infection on tomato roots appears as lesions on the root surface that produce black microsclerotia (the ‘black dot’ in its name) (fig. 1). Infected plants will sometimes wilt with the lower and middle leaves of the plant turning yellow.
C. coccodes has a very large host range, which includes members of Solanaceae, Cucurbitaceae and Leguminosae, both crops and weeds. Nightshade (a solanaceous weed) can harbor the fungus, often without showing symptoms. The pathogen can survive in the soil for up to eight years as microsclerotia.
This pathogen causes problems only under poor growing conditions or when other pathogens are present. The disease most commonly occurs in greenhouses or high tunnels where there has been a continuous cropping of tomato for several years, resulting in very high levels of inoculum. Other conditions in high tunnels, such as high temperatures and water stress can predispose plants to infection and root rot by C. coccodes. These infected plants can at times show few symptoms, but still result in yield reductions.
The disease can be prevented by growing in optimal conditions for tomatoes in the high tunnel. Deep plowing (12-15 inches, not easy to do in a high tunnel) degrades infected plant debris more rapidly and buries propagules both of which may help reduce fungal populations. Steam sterilization of the soil can greatly reduce disease incidence. Crop rotation is another tried and true management plan if growers can rotate out of any solanaceous, leguminous or cucurbit crops for 3-4 years – something most growers probably cannot financially do. Grafting is another possibility, but even grafted plants can still become infected with this disease. A particular root stock may be resistant or more tolerant of the disease, but this is something that would need to be checked before using. Small grains or corn can be used in rotation to reduce fungal populations.
Figure 1. Black dots (microsclerotia of C. coccodes, arrows) on tomato root