Our most economically important fruit disease is Botrytis fruit rot (BFR). BFR thrives under cool, wet conditions. Temperatures between 60F and 75F and wetness lasting over 16 hours are ideal for infection and disease development. These conditions are most likely to occur during the winter and early Spring, when fruit supply is low and prices are high. This makes BFR management most important during this time. It’s also when you’ll get the most bang for your buck from fungicide applications.

Realistic expectations

There are several fungicides that work well against BFR but it’s important to gauge expectations. We’ve conducted 16 fungicide efficacy trials over the last nine years. These trials show that our most effective fungicides will typically provide 40% to 60% reduction in disease over a no-fungicide control. BFR is a tough target. Other diseases such as powdery mildew can be controlled at much higher levels.

Remember that fungicides are protecting the bloom as well as ripe fruit. Infections during bloom can cause blossom blight and lesions on green fruit, but most infections remain latent until fruit reach maturity and are most susceptible. If conditions are unfavorable for further development when the fruit reach maturity, the infection remains latent. However, if favorable conditions persist or return at fruit maturity, the latent infection will develop into gray sporulation and fruit rot (Fig. 1). New infections on ripe fruit are also common. Since, strawberry plants are continuously flowering, this makes BFR management a continuous effort during periods of disease-conducive weather.

Consult fungicide efficacy tables

We work with University of California pathologists to publish fungicide efficacy ratings for common strawberry diseases. These can be found at https://ipm.ucanr.edu/legacy_assets/pdf/pmg/fungicideefficacytiming.pdf This publication is updated regularly and is based on many years of fungicide efficacy trials conducted under field conditions. It’s a reliable source of information. We publish a similar table in our Annual Field Day booklet: https://content-calpoly-edu.s3.amazonaws.com/strawberry/1/documents/FD2023/FungicideEfficacy_KB.pdf

Calyx architecture

Strawberry cultivars vary in their susceptibility to BFR. Much of this susceptibility is tied to calyx architecture. A large calyx that is appressed to the fruit surface will trap moisture and protect it from drying. Conversely, a reflexed calyx allows for maximum air movement and rapid drying (Fig. 2). Keep this in mind as you inspect fruit for BFR and select cultivars for your next planting.