Botrytis cinerea is the fungus that causes Botrytis fruit rot (BFR). Its spores are ubiquitous and readily dispersed through the air, with disease pressure largely influenced by weather conditions. Each year we conduct trials to determine the efficacy of fungicides against BFR). In order to have a successful trial, we need rainfall to coincide with the five-week treatment window. This works best with an early maturing cultivar because we are more likely to get fruit when conditions favor BFR development. This doesn’t guarantee success, but it stacks the deck in our favor (i.e., getting high levels of BFR).  However, this year was very different.  Despite the recent rains, there was no rainfall and unusually high temperatures (>80°F) during the trial.

So, what effect do dry and warm conditions have on BFR and what can we learn from a trial conducted during such unusual conditions?

Our BFR efficacy trials are conducted at Cal Poly using cultivar Fronteras, a short-day UC cultivar. When plants begin producing an average of one flower per plant, we initiate a series of five weekly fungicide applications using a handheld backpack sprayer. As flowers mature into ripe fruit, they receive a weekly fungicide application.

Every trial is a little different, not only because of the different fungicides evaluated, but more importantly, the environmental conditions are different. What made this trial unique was the absence of rain or prolonged wet conditions, combined with unusually high temperatures (Fig. 2).

Given the unique weather during this trial, we decided to evaluate disease levels after only three of the five fungicide applications. We wanted to answer the question: under dry conditions, is it important to protect the flowers to manage BFR?

Fruit was harvested one day after the third fungicide application and evaluated for BFR incidence in the field followed by a postharvest evaluation six days after harvest (6 DAH). BFR in the field was less than 0.5% (5 decayed fruit in 1,000) with no statistically significant differences between the non-treated control and any fungicide treatment. Similarly, in the postharvest evaluation there was very low disease pressure in the non-treated (Fig. 3).

In other trials conducted under conditions more favorable for disease development, the non-treated control in postharvest evaluations typically shows 60–80% decay at 6 DAH, two to three times higher than what we observed in this trial.

How should these results be interpreted? When trials lack statistical separation among treatments, interpretation shifts from direct treatment comparisons to the broader experimental context. These results do not suggest that these fungicides are ineffective; rather, they indicate that the environmental conditions were not conducive to disease development, limiting the potential for measurable treatment effects. The lack of differences from the non-treated control suggests that, under these conditions, fungicide applications did not significantly contribute to BFR control because there was so little of it to control! This highlights the value of a risk-based approach in which applications can be reduced without compromising control, lowering costs and preserving fungicide efficacy for periods of higher disease pressure.

Additional results and final conclusions on this trial will be presented at our annual field day on July 23, 2026. Click here for more information about our annual field day. We hope to see you there!