Classic Botrytis data from 1987

Figure 1. Botrytis fruit rot (gray mold) showing typical gray sporulation beginning under the calyx and spreading towards the tip.

A lot of people never heard the word “epidemic” (the rapid spread of a disease), “pandemic” (spread of a disease over a wide area) or “epidemiology” (the study of how disease spreads) until the COVID pandemic. Plant pathologists have been trying to figure out what drives plant disease epidemics since the discipline began back in mid-1800s. In the mid-1980s plant pathologists at Ohio State University did some epidemiological research and looked at how wetness duration and temperature affect Botrytis infection in strawberry flowers and fruit. They found that of the five temperatures evaluated, the optimum for disease development in fruit is 68F (20C) and that wetness duration beyond 16 hours resulted in a sharp increase in disease incidence.

I find this a useful chart (Fig. 2). It shows the relationship between wetness duration and temperature as it relates to Botrytis fruit rot. So I took the figure that Bulger et al. published in 1987 and redrew it using colors and cleaner lines so I could use it for presentations.

Figure 2. Effect of wetness duration and temperature at flower inoculation on the predicted proportion of ripe strawberry fruit infected by Botrytis cinerea.

Modified from: Bulger, M. A., Ellis, M. A. and Madden, L. V. 1987. Influence of temperature and wetness duration on infection of strawberry flowers by Botrytis cinerea and disease incidence of fruit originating from infected flowers. Phytopathology 77:1225-1230.

Fortunately for us in California, it’s rare to have the kind of leaf wetness that results in major outbreaks of Botrytis fruit rot, but in certain years, it can get bad. In 2016 we had periods of several consecutive days when it stayed wet. During that period, I saw as much as 80% fruit rot in the field. Our own research at the Strawberry Center shows us that in the absence of fungicides, we will typically see less than 5% Botrytis fruit rot in a typical (i.e., dry) year.

And how to manage Botrytis fruit rot in dry years is a story for a future post. Stay tuned…