Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu and Karen Rane, Director UMD Plant Diagnostic Laboratory
In the last few days we have received tomato samples that have the same unusual disease called Tomato pith necrosis. Tomato pith necrosis is caused by the soilborne bacterium Pseudomonas corrugata. Pith necrosis has occurred infrequently in Maryland over the past few decades. The disease usually is found in early planted tomatoes when night temperatures are cool, but the humidity is high, and plants are growing too rapidly because of excessive nitrogen application. Once night temperatures warm up, the plants usually outgrow the problem. We have had an early spring, which has allowed many growers to plant their crops 2-3 weeks earlier than normal. We then had cool nights in May and at times 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 young leaves and shoots, followed by wilting of the infected shoots in the upper part of the plant canopy (Fig. 1). This wilting is usually associated with internal necrosis at the base of the stem. 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 the pith will be discolored, and may have hollow areas (Fig. 3). There is often prolific growth of adventitious roots in the stems with discolored pith, and the stems may appear swollen.
There is not much 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. Using plant activators such as acibenzolar-S-methyl (Actigard) have resulted in 55% disease reductions, but applications must be started before symptoms appear. 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. Whole plant symptoms of tomato pith necrosis
Figure 2. Splitting of the main stem and darkened pith caused by tomato pith necrosis
Figure 3. Discolored pith and prolific adventitious root growth cause by tomato pith necrosis