Epiphytic Survival of Erwinia tracheiphila on Muskmelon (Cucumis melo L.)

Erwinia tracheiphila, the causal agent of bacterial wilt of cucurbits, is transmitted by striped (Acalymma vittatum) and spotted (Diabrotica undecimpunctata howardi) cucumber beetles. Transmission occurs when infested frass with E. tracheiphila is deposited on plant surfaces with fresh feeding wounds. However, it is unclear whether the pathogen can survive as an epiphyte on leaves. Experiments were conducted in controlled environments to monitor E. tracheiphila survival on muskmelon (Cucumis melo) leaves under various temperature and moisture conditions. In the first experiment, muskmelon seedlings that had been spray inoculated with a rifampicin-resistant strain of E. tracheiphila were incubated at 10, 15, 20, 25, 30, or 35°C (±2°C) at ≥95% relative humidity, and E. tracheiphila populations were monitored for 72 h. In the second experiment, E. tracheiphila was monitored during alternating 12-h wet and dry periods, or continuous wet or dry conditions for 48 h at 20°C. Survival of E. tracheiphila on wet muskmelon leaves depended on temperature (P < 0.01), with the greatest survival at 10 and 15°C and least at 30 and 35°C. Leaf wetness also impacted survival; an initial 12-h dry period resulted in a 1,000- to 10,000-fold reduction in population size, followed by stabilization of the surviving population. These results demonstrate that E. tracheiphila can survive on muskmelon leaves under a wide range of environmental conditions, suggesting that epiphytic populations might serve as a reservoir of inoculum for infections.