Importance of overwintered early blight-infected potato vines in Maine

Abstract

Much has been published on laboratory techniques for induction of sporulation of the potato early blight fungus, Alternaria solani (Ell. and Mart.) Jones and Grout. Douglas and Pavek (2, 3) briefly discuss this work in a report outlining a pure culture method they use to prepare inoculum for field resistance tests. Barclay, et al (1) and Venette and Harrison (9) have used a filter paper method described by Lukens (7) for the purpose. Conversely, early workers such as Folsom and Bonde (4) and Gratz and Bonde (5) favored use of infected vines as their inoculum source. The purpose of the report is to reaffirm the importance of overwintered diseased vines for both natural and artificial dissemination of early blight. Commercial Farms--Maine growers following short rotations and especially those who practice continuous cropping were found to have an increasing problem with early blight. Examination of their fields yielded dead vines carrying spores of the causal fungus. Tests showed that these spores were viable and that they readily produced infection on potato. Spore-trapping--Simple weather vane-type spore traps were used to approximate the time when primary inoculum first appeared (6). As expected, the most frequent trapping of spores occurred in a field having a high incidence of early blight the previous year. Spores were found around the first week in July each year that trapping was conducted. Fungicide Tests--In recent years tests have been conducted in Maine to determine the efficacy of fungicides and application schedules for control of early blight (8). During the course of this work it was found that dried, diseased vines, were an excellent source of primary inoculum. Each year at the conclusion of a field experiment but prior to application of a chemical defoliant, infected vines are collected and dried in the greenhouse. The dried vines are stored in an open shed through the winter. When needed the following year, they are pulverized through a coarse sieve. Inoculum thus prepared is sprinkled on potato foliage in the evening during a period of light rain or when dew is present. This procedure results in heavy and uniform infection provided that the material is distributed evenly.