Developing Damage and Economic Thresholds for Foliar Disease Management in Perennial Plantings of Strawberry

Introduction - Leaf spot, leaf blight, and leaf scorch are foliar diseases of strawberry found commonly in perennial plantings throughout North America. The effect of these diseases on strawberry production is suspected to range from reduced yields to shortened production life. A three-year study was implemented to determine to what extent these diseases impact yield and the production life of a planting, and to define when it is economically feasible to manage these diseases.

The objectives of this study are:

1) Determine the effect of leaf spot, leaf blight, and leaf scorch on the rate of photosynthesis in strawberry leaves under greenhouse conditions;

2) Determine the effect of leaf spot, leaf blight, and leaf scorch on strawberry yield in perennial matted-row field plantings of strawberry;

3) Develop an economic basis for managing foliar disease based on the relationship among disease severity, photosynthetic rate, and strawberry yield.

This study is funded through the NY IPM program, the North American Strawberry Growers Association, and through Federal Hatch funding.

Accomplishments: This study has been more difficult to conduct than anticipated. Our greenhouse studies were hampered because we were unable to generate sufficient disease with artificial inoculation of leaf blight or leaf spot. We eventually settled on focusing efforts on leaf scorch because we could easily produce symptoms with artificial inoculations. Preliminary results showed that photosynthesis declined rapidly and approached zero as leaf scorch severity increased. This implies that moderate to heavily infected leaves are contributing very little to the development of the plant and would presumably have a negative impact on yield. As we moved forward with this study, new information in regards to measuring photosynthesis has come to attention, and our research plan had to be adjusted accordingly. Last winter, however, we made some significant headway in this experiment, but it is too early to report accomplishments.

We had equally discouraging problems with the fields study. We attempted to produce a gradient of disease severities in experimental plots in Ithaca and Geneva by altering management strategies. We also hoped to "select" the various diseases through our choice in cultivar; i.e., to generate leaf spot we selected a variety very susceptible to leaf spot, etc. In short, there were no differences among treatment plots with respect to yield, crown or biomass data. This is partially attributed to the difficulties in generating this disease gradient across treatment plots as well as limiting natural infection to only one pathogen per cultivar. This is proving to be a much more difficult task than we anticipated. Moreover, in addition to the foliar diseases under study, powdery mildew and angular leaf spot are appearing with greater frequency. To circumvent future problems, we will modify our field trials to look at the cumulative effect of foliar disease, rather than focusing on any particular pathogen or set of pathogens, and we will alter our gradient to generate 3 rather than 7 levels of foliar disease, which we believe we can accomplish by altering our fungicide regimes.