Question of the Week – Phytotoxicity

What caused the spots on these watermelon leaves?

These mysterious brown spots were observed in several fields that had recently been treated by an airplane. At first glance, the small brown lesions resembled symptoms of a foliar disease. However, several observations suggested a non-pathogenic cause. 

The spots were only found in fields that had received aerial applications. Within affected fields, the lesions were widespread and uniformly distributed across both the field and individual leaves. In addition, many weed species within the fields and along field borders exhibited similar symptoms and distribution patterns. 

Closer examination revealed no signs of sporulation, and the lesions lacked the yellow halos commonly associated with bacterial leaf spot diseases. To further investigate the cause, leaf samples from two affected fields were collected and submitted for diagnostic evaluation. 

Laboratory examination indicated that the lesion pattern was consistent with spray injury. Leaves were placed in a moist chamber overnight and reexamined the following day. While some lesions enlarged slightly and a few additional spots developed, no sporulation or other evidence of a disease-causing pathogen was observed. 

The explanation for the mysterious leaf spots turned out to be relatively simple. While plants do not have immune systems like humans, they can respond to abiotic stress and certain chemicals with a range of physiological and morphological changes. In some cases, pesticides or spray additives can trigger a delayed stress response, with symptoms appearing several days after application. These symptoms often include small brown lesions or spots on leaves. As part of the plant’s natural defense process, affected cells may die (necrosis) to isolate the injury and prevent further damage to surrounding tissue. 

In this instance, the grower was advised to continue monitoring affected fields for any changes in symptom development and to communicate their observations with the aerial applicator. At the time of evaluation, the injury appeared cosmetic, affecting only the foliage, and no modifications to the current spray program were recommended. 

This week’s question is from Anna Sara Hill, Lowcountry Fruit and Vegetable Extension Agent.