RALEIGH, N.C. — Winter is a critical period for setting the foundation for healthy strawberry production. Although insect and weed management begin in the fall, and diseases are not usually observed until spring or summer, disease control must start well before symptoms appear. The key to winter disease prevention is minimizing plant stress and reducing the sources of pathogen carryover.
Strawberry pest management should start with controlling weeds and insects. Apply a pre-emergent herbicide in the fall to prevent early weed establishment, then hand weed in early spring when stems are still fragile. Monitor for pests such as aphids and spider mites, especially when using row covers, and address issues quickly to prevent plant stress.
Plasticulture strawberries do not enter dormancy, which makes them more vulnerable to environmental stress. Stressed plants are more susceptible to disease in the spring, so maintaining healthy plants through winter is essential. Irrigate during dry periods to prevent drought stress. As temperatures gradually cool in the fall, plants naturally increase their cold resistance. When temperatures fall below 20℉, protect plants with row covers to prevent winter injury.
Many strawberry pathogens overwinter inside infected plants and in intact dead leaves. Removing symptomatic plants, damaged foliage, debris, and weeds reduces the amount of pathogen inoculum that will be present in the spring. This sanitation step helps lower disease pressure during the growing season.
Generally, preventative fungicide applications should be reserved for the spring when flowering begins, although exceptions exist.
Does a winter fungicide application make sense for your farm? Let’s look at the time from December through early March. The following risk factors can help determine whether a winter fungicide spray is appropriate for your farm.
Visible Disease Symptoms. Some pathogens, if present, can produce leaf spots on transplants at planting or develop symptoms during fall establishment. Did plants appear diseased at time of planting? If symptoms were visible, winter fungicide sprays may be necessary. Scout rows carefully during the winter months and examine older foliage for leaf spots. Neopestalotiopsis (Fig. 1) can produce noticeable leaf lesions in the fall. If spots are present, fungicides should be applied to limit further spread, and hand removal of old, senescent leaves should be planned for late winter before new spring growth emerges.
Figure 1. Tan lesions with reddish-brown margins on Neopestalotiopsis-infected strawberry plant
Plant size and crown density. Large plants with dense canopies that hold moisture throughout the day, especially during warm winter spells, are at higher risk for Botrytis crown rot (Fig. 2). When these conditions occur, a winter application targeting crown rot may be the most strategic use of the single permitted application of Rovral (iprodione), FRAC group 2, in strawberries. After this point, spring fungicide programs should take over once flowering begins. Do not wait for Botrytis symptoms to develop. Fungicides are preventative, and the Botrytis pathogen is always present in your field, which means disease can progress quickly if conditions are favorable.
Figure 2. Gray mold spores growing on Botrytis-infected strawberry plant
Row covers. If row covers will be used in winter to promote plant growth, consider a fungicide application prior to covering. Covers should be placed when both the cover and the plants are dry, and they should be monitored periodically for diseases by pulling back the covers for visual inspection. Row cover management in winter should focus on both plant growth and disease risk. Warm, wet conditions beneath the cover can create a “moist chamber” that favors disease development. During warm winter spells, monitor closely and pull covers back if needed to reduce disease pressure. In late winter and early spring, row covers serve as freeze protection and should be applied ahead of forecasted freezing temperatures and then removed soon after conditions warm up. It may be necessary to apply and remove row covers multiple times to balance freeze protection with disease prevention.
Daisy Ahumada, Caleb Bollenbacher, and Bill Cline, Entomology & Plant Pathology, NCSU
–Daisy Ahumada, N.C. State University
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