Validation of the Strawberry Advisory System in the Mid-Atlantic Region

Anthracnose fruit rot (AFR) and Botrytis fruit rot (BFR) are primary diseases affecting strawberry (Fragaria × ananassa), which typically drive fungicide applications throughout the growing season. The Strawberry Advisory System (StAS), a disease forecasting tool, was originally developed in Florida to better time the fungicide sprays by monitoring AFR and BFR infection risk based on leaf wetness and temperature input in real-time. Thirteen field trials were conducted in Maryland and Virginia between 2017 and 2019 to evaluate the StAS performance in the Mid-Atlantic region. As a result, 55, 18, and 31% fewer sprays were recorded on average in the model-based StAS treatment compared with the grower standard treatment in 2017, 2018, and 2019, respectively. Marketable yield, as well as AFR and BFR incidence, were largely comparable between the two treatments. However, poor disease control occurred during the StAS treatment in four trials in 2017, presumably because of a missed fungicide spray during a high-risk infection event and attributable to heavy rainfall that led to impassable fields. The implementation of the StAS may be further challenged by the employment of floating row covers that are essential for growing strawberries in plasticulture systems in open fields in the Mid-Atlantic region. Preliminary results indicated that row covers can alter canopy-level microclimatic conditions, possibly increasing the risk for disease occurrence. Overall, the StAS can be a valuable tool for Mid-Atlantic growers to control AFR and BFR, but sprays may need to be promptly applied when consecutive or heavy rainfalls are predicted, especially for highly susceptible cultivars. Complications in disease forecasting and management arising from the use of row covers need to be further addressed in this region because of its highly diverse climate.

Resistance to Azoxystrobin and Thiophanate-Methyl Is Widespread in Colletotrichum spp. Isolates From the Mid-Atlantic Strawberry Fields

Multiple Colletotrichum species have been found to be responsible for strawberry anthracnose, and prevalence of each species seems to vary by regions and/or host tissues. In this study, a total of 200 Colletotrichum isolates were obtained from different strawberry cultivars displaying anthracnose symptoms in the mid-Atlantic fields. Analysis of g3pdh, tub2, and/or internal transcribed spacer sequences revealed four Colletotrichum species, including C. nymphaeae, C. fioriniae, C. siamense, and C. lineola. C. nymphaeae was the predominant species, representing 90% of all isolates collected. This species was found from all strawberry organs/tissues examined, whereas C. siamense and C. fioriniae were limited to the crown and fruit, respectively. Further, all Colletotrichum isolates were screened for resistance to azoxystrobin in vitro, and all C. siamense isolates were additionally screened for resistance to thiophanate-methyl. The overall frequency of resistance to azoxystrobin and thiophanate-methyl was 48 and 67%, respectively. G143A in the cytochrome b gene was found in all C. nymphaeae and C. siamense isolates with high level of resistance, with EC₅₀ >100 µg/ml, while F129L was found in two of the five C. nymphaeae isolates with moderate resistance, with EC₅₀ values ranging from 2.6 to 7.8 µg/ml. All C. fioriniae isolates tested were found to be less sensitive to azoxystrobin, with EC₅₀ values ranging from 9.7 to 14.4 µg/ml, despite no mutations detected in the cytochrome b gene. Moreover, E198A in tub2 was linked with C. siamense isolates resistant to thiophanate-methyl (EC₅₀ >100 µg/ml). These results revealed that resistance in Colletotrichum spp. to primary fungicides is widespread in the mid-Atlantic strawberry fields.