This season I’ve received a lot of emails and phone calls concerning fruit crops showing symptoms of decline often associated with diseases. It’s worth noting that many of the diagnoses I’ve offered have been linked to drought stress. With an excessive heat warning in south central Wisconsin this week, I think it’s important to offer a reminder about the tendency to mistake drought or heat stress for symptoms of disease. Also, exercising care when applying chemical treatments like fungicides during hot spells is essential. An additional observation I’ve made this summer is the occurrence of phytotoxicity, which can occur when fungicides are applied during very hot conditions.

Prolonged exposure to temperatures over 90°F leads to slowed plant growth and signs of stress. Heat stress in plants can manifest differently based on factors like plant type or maturity.

Symptoms of heat stress in fruit crops may show up in the form of:

• Leaf cupping or rolling
• Wilting
• Dry leaf margins
• Premature fruit drop
• Reduced fruit set
• Delayed ripening
• Stunting
• Sunscald

While numerous indicators of plant stress may resemble those linked to plant diseases, there are several important ways to differentiate an abiotic vs. biotic issues.

• Symptom Appearance: Stress often causes general changes in plant growth or color, such as wilting or stunting, while plant diseases tend to be more specific with spots or lesions, or growth abnormalities.
• Symptom Distribution: Stressed plants often display a consistent or uniform pattern in the field, and they can usually be found across an entire area or down a row; plant diseases are usually irregular, and symptoms may not be uniform, in fact some plants may be more affected than others.
• Symptom Progression: Plant stress symptoms can develop slowly and last as long as the stressor is present, sometimes plant stress can be reversed when the stress is no longer present; plant disease development depends on conducive environmental conditions and under these favorable conditions (ex. high moisture) diseases can progress quickly.
• Pathogen presence: Look for signs of the pathogen (ex. fungal growth or bacterial ooze) for plant diseases.
• Systemic vs. Localized Symptoms: Plant stress is often systemic or affecting a large portion of the plant; some plant diseases can be systemic or localized (ex. lesions).

Importantly, plant stress can also increase susceptibility to diseases, and this can sometimes complicate diagnosis. Keep in mind the points above and remember what conditions are considered conducive to the pathogens that may affect your crop. Whether you are dealing with plant stress or plant disease, a diagnosis is the first step in determining your management plan.

UW Plant Disease Diagnostic Clinic: https://pddc.wisc.edu/

This article was posted in Disease and tagged disease, heat stress, Leslie Holland, plant disease, plant stress.