Watermelon Seedling Diseases in the Greenhouse

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu and Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

Bacterial Fruit Blotch
Bacterial Fruit Blotch (BFB)
of watermelon, which is caused by the bacterium Acidovorax avenae subsp. citrulli, produces large olive green to brown water-soaked lesions on fruit, making them unmarketable. Symptoms of BFB on seedlings begin with water-soaked areas on the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.

If the bacterium is present, conditions in transplant production greenhouses are highly favorable for the development of BFB symptoms and the spread of disease. Good practices for greenhouse transplant production are to disinfect surfaces before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 10,000 seeds. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols, so keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. If BFB is suspected, collect a sample and submit it to your Extension educator, or specialist. Destroy all trays with symptomatic plants. Remove adjoining trays to a separate isolated area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled bactericide and the applications continued until the plants are transplanted to the field.

When receiving shipments of transplants, inspect them carefully for symptoms and get a diagnosis if symptoms are observed.

Figure 1. Water soaked appearance of the lower surface of the cotyledon infected with bacterial fruit blotch.

Figure 2. Watermelon transplant with bacterial fruit blotch. Note the yellow halos around the necrotic lesions.

Other Bacterial Diseases in the Greenhouse
Angular leaf spot (ALB)
, which also is a bacterial disease, looks similar to BFB. This “look-alike” disease occurred in Delmarva’s greenhouses several years ago. Symptoms of angular leaf spot are a chlorotic halo and may appear “shiny” (due to bacteria on the lesion surface). Small irregular lesions expand and become angular. On watermelons the borders are chlorotic. Older lesions may turn brown, dry and tear to produce a tattered appearance.

ALB also may be seedborne. In addition it has a wide host range and can also survive as an epiphyte on several weeds. The pathogen spreads from plant to plant in splashing rain, irrigation, or mechanically (such as on hands, windblown sand, or equipment). There are several bacteria (Pseudomonas viridiflava, P. syringae pv. lachrymans, and possibly others) that cause similar symptoms and vary in their ability to cause damage.

It is important to have the disease identified. The symptoms look similar to anthracnose. However the agrichemicals used to manage the two diseases are different. If angular leaf spot is confirmed in the field, applications of fixed copper plus mancozeb will minimize spread. Also avoid working field when foliage is wet.

Figure 3. Upper and lower surface of leaf with symptoms of angular leaf

Figure 4. Angular leaf spot. Note the angular tan appearance of lesions, and the “shine” on the cotyledons.

Fungal Diseases in the Greenhouse
The fungal diseases gummy stem blight, Alternaria leaf blight, anthracnose, and Fusarium wilt can also be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants.

Figure 5. Gummy stem blight infected transplants occur as clusters in an area around the initial infected seedling (foci).

Although I have not seen Fusarium wilt infected transplants in local commercial greenhouses, it has occurred in other states. Symptoms are wilted seedlings that may remain green or become chlorotic (yellow). This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.

Bottom line: If the seedlings appear diseased, identification of the problem is critical. Do not ship any trays containing plants with disease symptoms.