Watermelon Diseases

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

Over the past week the Plant Diagnostic Clinic has received numerous watermelon samples with disease symptoms. The following diseases have been confirmed by Nancy Gregory:

Alternaria Leaf Blight
Caused by the fungus Alternaria cucmerina, is a pathogen that can occasionally damage cucurbits in Delmarva. The fungus can survive on decaying plant material in the soil in the soil for up to 2 years and is dispersed from plant to plant by rain and windborne spores. Small dark spots with yellow halos are often first observed on older leaves. Over time, lesions can expand and form large necrotic spots with a characteristic “target-like” appearance. As the disease develops leaves eventually die and result in a general decline of plant health (Figure 1). Damage is greatest following wet weather and temperatures between 68 and 90°F. Infection is greatest when temperatures are between 59° and 70°F with at least 8 hours of continuous leaf wetness. In terms of management, growers should follow good sanitary practices and remove crop debris after harvest. Tillage will help bury and facilitate the decomposition of residue. Rotation out of cucurbits for 2-3 years will further reduce inoculum levels. There is some level of resistance in newer cultivars, but this resistance is believed to be only slightly better than older cultivars. Minimizing overhead irrigation will further help minimize potential outbreaks. It is important for growers apply preventative fungicides on a 7 to 10-day schedule and not skip applications. Labeled fungicides should prevent outbreaks of this disease if applied properly. When vines begin to run, alternate or tank mix chlorothalonil or mancozeb among the fungicides containing different modes of action. For example: QoI- Cabrio, Quadris, Reason; SDHI-Fontelis, Endura. Examples of premixes that will provide control include Luna Experience, Luna Sensation, Merivon, Pristine, Switch and Quadris Top. Resistance to QoIs can occur in Alternaria spp. , so if you have an issue with fungicide resistance development avoid applying products with a QoI component, this includes premix fungicides.

A watermelon leaf with late stage Alternaria leaf blight


Figure 1. A watermelon leaf with late stage Alternaria leaf blight. Sanitation, crop rotation, and maintaining a good preventative fungicide program should control disease outbreaks.

Anthracnose is a fairly common disease in Delaware that seems to be increasing throughout cucurbit production areas in the United States. Yield losses can approach 100% in some cases. Caused by Colletotrichum obiculare, this disease can affect all above ground plant parts, including fruit. The fungus survives in fields on crop residue and also can be spread from seed obtained from infected fruit. Spores require 24 hours of 100% relative humidity to infect, and infection is greatest when temperatures are between 72-81°F. Initial symptoms show up 4 days after infection. The disease starts as small circular lesions that gradually expand on leaves to produce large tan or brown spots that can fall out from the leaf or crack with age. Foliar lesions also tend to follow leaf veins. On fruit, lesions can grow to several centimeters in diameter, and often are cracked. Under periods of high humidity, masses of pink gooey spores can be seen in and on fruit lesions.

Anthracnose on a watermelon leaf


Figure 2. Anthracnose on a watermelon leaf. Note the cracking of lesions. Lesions also tend to follow leaf veins. Image obtained from the image archives at www.Bugwood.org.

There are excellent sources of resistance to anthracnose, and growers should consider planting certified disease-free seed of resistant varieties when possible. If anthracnose is an issue in a particular field, growers should consider rotating away from cucurbits for at least 2 years. Tillage and debris removal may help reduce inoculum. Minimizing overhead irrigation will further help minimize potential outbreaks. Protective fungicides will help with disease control if applied properly and on a regular schedule. Begin sprays when vines begin to run (or earlier if symptoms are detected). Alternate chlorothalonil or mancozeb with one of the following and rotate: Pristine, Quadris, Cabrio, Reason, Inspire Super, Quadris Top, Luna Sensation. As with Alternaria, QoI resistance is possible. If resistance to QoIs has been detected in your area, do not use products that contain a fungicide with a QoI mode of action, including premixes.

Gummy Stem Blight (GSB)
GSB is another somewhat common disease in Delaware that can cause severe yield losses. This disease is caused by Didymella bryoniae , another pathogen that survive in residue and can infect plants at any point in growth. Many times GSB can be found early in transplants because it also can be transmitted in seed. Disease if favored during periods of wet weather and temperatures around 75-77°F. Only 1 hour of free moisture is required for infection, but disease progress is determined by the length of free moisture. Initial disease symptoms can be observed within 3-4 days of infection. On older plants circular tan lesions develop, often starting at the leaf edges. As the lesion expands it often follows the leaf veins. In stems or vines lesions appear slimy or water soaked and are orange in color. Occasionally an orange-red exudate is associated with vine or stem lesions (Figure 3). On fruit, lesions start out as small, water-soaked spots that expand and can exude an orange-red material. With age spots become sunken. Often black specks appear throughout infected areas.

A stem of watermelon showing the reddish exudate associated with GSB


Figure 3. A stem of watermelon showing the reddish exudate associated with GSB. Photo from Zhou, X.-G., and Everts, K. L. 2008. Integrated management of gummy stem blight of watermelon by green manure and Melcast-scheduled fungicides. Online. Plant Health Progress doi:10.1094/PHP-2008-1120-01-RS.

To manage GSB, growers should ensure that they are planting certified, disease-free seed. Symptomatic transplants should not be planted. In fields with a history of disease a 2-4 year rotation out of cucurbits is recommended. Minimize overhead irrigation. No cucurbit varieties are available with resistance to GSB. Fungicides will provide good control if applied properly and according to label directions. We have QoI resistant GSB in the area, so fungicides containing a QoI are not recommended. On a 7 to 10-day schedule, alternate chlorothalonil with tebuconazole, or Fontelis. Premix fungicides should be tank mixed with chlorothalonil before application. Examples include Pristine, Switch, Inspire Super, and Luna Experience.

Remember with any fungicide application that the label is the law.

To help manage diseases such as Anthracnose, growers are encouraged to sign up for MelCast, a weather based program that helps you with fungicide application decisions. It is similar in many ways to the late blight programs used throughout the region. The following message is from Kate Everts, Vegetable Pathologist at the University of Delaware and the University of Maryland:

“MelCast is a weather-based spray advisory program for watermelon developed at Purdue University. The program uses hours of leaf wetness and temperature during leaf wetness periods to determine when a fungicide should be applied. Weather information is “fed into” the program three times per week and the program indicates how favorable weather is to the development of gummy stem blight or anthracnose. The output of the program is an “environmental favorability unit” (EFI) for each day. The EFI values are added together. Once the threshold of 30 EFI is reached, a fungicide application is recommended. In Maryland and Delaware we have reduced the time between applications (i.e. reduced the EFI value that triggers a spray recommendation). After the fungicide application, begin adding the EFI again. If two weeks elapse and you have not accumulated 30 EFI, spray anyway. Also, add 2 EFI for each overhead irrigation event. We currently run MelCast for three locations in Delaware (Coverdale Crossroads, SE Laurel, SW Laurel) and five locations in Maryland (Galestown, Hebron, Salisbury, Waldorf and Woodbine).

To use MelCast on your farm, please call Karen Adams at (302)856-7303 and give us your name and Fax number or e-mail address.”