A new bacterial disease of cucurbits in N.Y. caused by Serratia ureilytica
Kensy D. Rodriguez-Herrera, Sarah J. Pethybridge, Steve Reiners, Brian Nault, Bryan Swingle and Christine D. Smart, Cornell University
Cucurbit yellow vine disease, caused by the bacterium Serratia ureilytica, has been found in the midwestern and eastern US. It was first detected in New York three years ago. This disease is transmitted by the squash bug (Anasa tristis), a serious pest of cucurbits. The most common hosts of this pathogen include melon, squash, zucchini, pumpkin, and watermelon.
Symptoms begin with plant decline (Fig. 1), interveinal yellowing leaves (Fig. 2), followed by the yellowing of entire vines. Plant wilting may begin as soon as two weeks after the first symptoms appear.
Other symptoms include stunting and plant collapse (Fig. 3A). The bacterium moves systemically in the vascular tissue of the plants. The vascular tissue of affected plants can sometimes have a honey-brown color when looking at the cross-section of the stem at the crown of the plant.
Symptoms may be similar to those caused by bacterial wilt, which is caused by the bacterium Erwinia tracheiphila and transmitted by the striped cucumber beetle (Acalymma vittatum) and spotted cucumber beetle (Diabrotica undecimpunctata). Although both diseases can be seen as wilting and scorching of leaf margins, the main difference between them is the yellow appearance in CYVD compared to bacterial wilt (no yellow appearance) (Fig. 3B).
Disease cycle
The bacterium overwinters in the adult squash bug. Early infection of cucurbit crops is caused by squash bugs feeding on the plant (Fig. 4). Other potential vectors are striped cucumber beetle and spotted cucumber beetle. Squash bugs also acquire S. ureilytica while feeding on infected plants, and infected insects can transmit the bacterium to other cucurbit plants as they feed. The acquired bacteria can persist in the squash bug gut through periods of molting and/or hibernation and serve as primary inoculum for the next cropping season. When adult squash bugs emerge from hibernation (typically early to mid-June), they feed on cucurbit plants and lay egg masses (Fig. 4) on the underside of leaves and other parts of the plant.
After eggs hatch, squash bugs go through a nymphal stage before becoming an adult (Fig. 5). This process can take from four to six weeks. The climate in New York usually prevents more than one generation of squash bugs per year. The disease cycle can be seen in Figure 6.
Management strategies
Current disease management practices are aimed at controlling the squash bug vector. Although squash bugs are challenging to manage, insecticide use is one of the most effective strategies for control. Check with your local Cornell Cooperative Extension office regarding registered insecticides. Squash bugs usually aggregate on the underside of leaves or at the base of cucurbit plants (Fig. 4), allowing them to escape from foliar-applied insecticides. Although plastic mulch can help control other pests, it gives the squash bug a place to shelter, which can increase its population.
Therefore, managing the first generation of squash bugs during their nymphal stage is crucial to reducing the size of the adult population. Cultural practices to control the squash bug include crop rotations with non-cucurbit plants or at a sufficient distance from where previous cucurbits were, removing crop debris and old fruit that will not be harvested, so that the overwintering population can be reduced. Using Hubbard squash as a trap crop on plot edges can help reduce disease in the main plot.