Control of Eutypa Dieback
Objectives: The overall objective of this research is to develop more effective methods for the management of Eutypa dieback. There are several specific aspects to this broad objective, including: 1. Evaluation of various fungicides for pruning wound protection. 2. Determination of the duration of susceptibility of wounds and the mechanism by which they become less susceptible with time. 3. Evaluation of natural pruning wound colonizers as wound protectants (biological control). 4. Investigation of the relationship between vineyard age and disease incidence. Samjuaxy.: In four field experiments, only two fungicides, Benlate and Nustar, were consistently very effective. In general, the fungicides were more effective when wounds were inoculated one day after fungicide treatment, compared with inoculation after two weeks. Wound colonizing microorganisms were screened in the laboratory and the field for their potential to inhibit Eutypa. The ten isolates tested in the field were less effective in 1991 than in 1990. However, the biological control agents were generally more effective when inoculation was delayed. Two of the biological control agents, Fusarjum lateritium and Cladosporium herbarum. were as effective as benomyl when inoculated with Eutypa two weeks after treatment. Wound susceptibility experiments from 1991 indicated that wounds made in December were susceptible much longer than one month. Wounds made later in the dormant season were susceptible for a shorter duration, depending primarily on temperature. In both 1990 and 1991, accumulation of degree-days after pruning was strongly related to xylem wound response and populations of epiphytic microorganisms on the wounds. These factors led to decreased susceptibility. Degree-day accumulation may be used to predict wound susceptibility. These data support the practice of pruning when warm weather is likely to follow (late pruning). Different grape cultivars appear to have different levels of susceptibility to Eutypa in the field. We have used ascospore inoculations and mycelial inoculations, but have not detected any strong differences in susceptibility among cultivars. In order to characterize the relationship between vineyard age and incidence of disease, we surveyed, in 1991, 12 Chenin blanc vineyards ranging in age from 8 to 24 years. Results were very similar to those obtained for French Colombard in 1990. Vines less than 10 years of age had little disease. Disease incidence increased rapidly between 10 and 20 years, and stabilized after 20 years of age, with nearly all vines, but less than half of the spurs diseased.