Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
In the last week tomato fields in Maryland were found with the disease tomato pith necrosis. Just about all the problem tomatoes were from early planted fields. Tomato pith necrosis is caused by the soilborne bacterium Pseudomonas corrugata. Although in the past this disease occurred sporadically in Maryland, over the last few years it is appearing more frequently. Tomato pith necrosis usually is found in early planted tomatoes when night temperatures are cool, but the humidity is high, and often plants are growing too rapidly because of excessive nitrogen application. We have had a spring/early summer with some cool nights and high humidity.
In the field, diseased plants occur randomly with initial symptoms often being seen as the first fruit clusters reach the mature green stage. Symptoms include chlorosis (yellowing) of lower, middle and even younger leaves (Fig. 1) followed by wilting of the infected shoots in the upper part of the plant canopy. This wilting is usually associated with internal necrosis at the base of the stem and black streaking may be apparent on the surface of the main stem, which often splits (Fig. 2). When the stem is cut open along its length or crosswise (Fig. 3) the pith will be discolored and may have hollow areas. There is often prolific growth of adventitious roots in the stems that have a discolored pith (Fig. 4), and the stems can appear swollen.
There is little that can be done for control of pith necrosis. The best practice is prevention by avoiding the use of excessive amounts of nitrogen in tomato, especially early in the season when nights are still cool. There is some evidence that the pathogen may be seedborne, but more research is needed on the epidemiology and management of this disease.
Figure 1. Beginning of pith necrosis- leaves anywhere on plant can turn yellow
Figure 2. Tomato stem with dark streak and lesions on its surface
Figure 3. Darkened pith of tomato stem
Figure 4. Prolific growth of adventitious roots on stem