Pseudomonas Leaf Spot Reported on Watermelon Transplants

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In the past week we have seen a few cases of watermelon transplants with leaf spot symptoms (Fig. 1). Examination of samples at the NCSU Plant Disease and Insect Clinic has revealed that the culprit of the symptoms is a Pseudomonas spp. We have not seen a lot of this disease in North Carolina, but states such as Florida and Georgia have reported occurrences of Pseudomonas leaf spot in watermelon in past years.

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Fig. 1: watermelon transplants infected with Pseudomonas leaf spot (Photo credit: NCSU Plant Disease and Insect Clinic)

Symptoms consist of leaf spots that appear dark, greasy and/or water soaked, and later become light tan in the center with a concentric ring patter in the lesion (Fig. 2). Lesions can occur in leaf margins, expand, and result in some defoliation (Fig. 3). Research indicates that cool, wet weather favors the disease, which explains why we are seeing some cases this spring. Work conducted in Florida also revealed that applications of copper and mancozeb every 7 days can delay disease symptoms until the weather becomes dry and warm. Buying certified seed and removing transplants with leaf spot symptoms are two ways to help reduce the amount of inoculum brought into the field. Rotating to a non-cucurbit crop in fields that had the pathogen and proper weed control are two additional ways to reduce the chances of repeat infections according to Florida researchers.

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Fig. 2: watermelon transplants infected with Pseudomonas leaf spot. Note the leaf spots that appear dark and later become light tan in the center with a concentric ring patter in the lesion (Photo credit: NCSU Plant Disease and Insect Clinic).

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Fig. 3: watermelon transplants infected with Pseudomonas leaf spot. Note the lesions in leaf margins that expand and later result in defoliation (Photo credit: NCSU Plant Disease and Insect Clinic).

If you think you have plants infected please contact your local Extension Agent and send photos and/or physical samples to the Plant Disease and Insect Clinic for confirmation. Observation of lesions under the microscope will reveal heavy bacterial streaming.

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Written By

Photo of Dr. Lina Quesada-OcampoDr. Lina Quesada-OcampoAssistant Professor, Plant Pathology (Cucurbits and Sweetpotato) (919) 513-3530 lina_quesada@ncsu.eduEntomology and Plant Pathology - NC State University
Posted on May 20, 2016
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