Watch for Phomopsis Leaf Blight in Strawberries

Phomopsis leaf blight was one of the most common leaf diseases observed in Ontario strawberry fields in 2003. The fungal pathogen, Phomopsis obscurans, can infect most above ground parts of strawberry plants including leaves, runners, petioles and fruit. Fruit infection is uncommon; symptoms of leaf blight are most obvious on leaves. Early symptoms- little circular reddish, purple spots on leaflets- look similar to common leaf spot. As the disease progresses, the spots expand along major veins and develop into diagnostic "V" shaped lesions with the widest part of the V at the leaf margins. Purplish -red lesions on leaf petioles can cause leaf dieback. Often, severe symptoms on leaves are not noticed until close to harvest time. Severely diseased plants may not yield well. Plants weakened from phomopsis may be more susceptible to winter injury.

The disease survives from year to year in infected leaf debris. Management practices include:

• Removal and burial of infected leaves during renovation.
  
• Use of tolerant varieties.
  
• Application of an effective and properly timed fungicide, such as Equal 65 WP. Unfortunately, Equal 65 WP applied in cold temperatures and cold weather can injure strawberry leaves.
  

Unfortunately, Phomopsis leaf blight occurs sporadically from year to year. Infection depends on weather conditions leading up to harvest. If we could predict when infection takes place, growers could target fungicide applications more precisely.

In a recent study¹ at Ohio State University, researchers looked at the influence of temperature, leaf wetness duration and leaf age on the infection of strawberry leaves cv. "Honeye" and "Earliglow". Their objective was to develop a prediction model for leaf blight. Disease incidence and severity were most influenced by the age of the leaf. The younger the leaf at the time of infection, the higher the disease observed four weeks later. Leaf wetness duration also significantly influenced disease. Longer leaf wetness periods resulted in higher disease. Surprisingly, relationship between infection and temperature on disease severity and incidence was minor.

The results of this study may help to explain the severity of leaf blight observed in 2003. Extended rainy periods in late May and early June were ideal for infection of the new leaves. In addition, overhead irrigation for frost protection resulted in more extended leaf wetness periods. Although August was very warm and dry, September and October were very wet, which resulted in more infections.

High levels of leaf blight may survive this winter and could result in higher disease pressure this coming spring. The disease prediction model developed in Ohio has not been tested in Ontario, however, results suggest that fungicides should be applied to protect new leaves, prior to a leaf wetness event greater than 5 hours, regardless of temperature. This will be particularly important during next spring and after renovation when plants are producing an abundance of new susceptible leaves.

Diagnostic V-shaped Leaf blight lesion on infected strawberry leaf.
Severe disease is often not observed until 3 weeks after infection.

¹ Nita, M., Ellis, M.A. and Madden, L.V. 2003. Effects of temperature, wetness duration, and leaflet age on infection of strawberry foliage by Phomopsis obscurans. Plant Disease 87:579-584.

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