Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu and Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Transplant production is in high gear on Delmarva and some problems, mostly abiotic, have been observed. Although these current problems are primarily abiotic, a refresher on common diseases is always good. In addition, because of our delayed spring, the weather may cause some transplants to be held longer than usual before planting, which isn’t optimal and will further stress the seedlings. If any seedlings appear diseased, identification of the problem is critical.
The most common fungal disease of tomato transplants in the greenhouse is gray mold, caused by Botrytis cinerea. However, leaf mold caused by Fulvia fulva and even late blight caused by Phytopthora infestans can occasionally be problems. All these diseases can be reduced by changing watering practices (by watering the base of the plants and early in the day to reduce moisture on the foliage). Also improving ventilation, venting, and heating (if outdoor temperatures are cool enough) to reduce humidity in the greenhouse will minimize disease.
Bacterial diseases of tomatoes include bacterial spot and speck. To reduce these diseases use certified and hot-water treated seed. Maintaining a good environment is critical to managing diseases. Additionally, there are organic and conventional fungicides that are labelled to use in the greenhouse. See the Delaware Commercial Vegetable Recommendation Guide for more information.
Transplants of squash, cucumber, watermelon and cantaloupe also may contract fungal and bacterial diseases. Reoccurring diseases here on Delmarva include Bacterial Fruit Blotch (BFB), which is caused by the bacterium Acidovorax avenae subsp. citrulli. 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 greenhouse transplant houses 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 watermelon 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 of cucurbits 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. 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. Therefore, it is important to have the disease identified. The symptoms look similar to anthracnose.
Figure 3. Upper and lower surface of leaf with symptoms of angular leaf spot.
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 clusters of diseased plants.
Although we 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).
An additional reason to be concerned about diseases that occur in a greenhouse on transplants is that, if planted, these transplants can introduce new strains of the pathogen into your field. For example, new strains or races of the Fusarium wilt pathogen can be introduced into an area on seedlings grown from infested seed. Recently, scientists in Florida have detected fungicide resistant isolates of Didymella bryoniae (the gummy stem blight pathogen) in transplant greenhouses. If planted to the field, fungicides used for managing gummy stem blight may be ineffective on the crop.