Jake Jones, Extension Agriculture Agent, Kent County; jgjones@udel.edu
Gummy Stem blight is caused by Stagonosporopsis spp. and can occur on cucurbit crops and seedlings. It can be introduced from infected seed or seedlings, highlighting the importance of greenhouse and field sanitation. Gummy stem blight can cause symptoms on the leaves, stems, and vines and is also called black rot when fruits are infected. Growers should be able to recognize gummy stem blight symptoms on seedlings before transplanting them into clean fields. Symptoms include water soaked stems (Figure 1), chlorotic leaf margins, and necrotic lesions on cotyledons and leaves. The necrotic lesions are often chocolatey brown and the majority reach the leaf margins. Concentric rings can be found within the lesions. Diagnostic signs can be seen with a hand lens and are pycnidia, the asexual fruiting body or pseudothecia, the sexual fruiting body. They form in the center of lesions first and can be found in the final stages of infection, as the pathogen is necrotrophic. In order to limit transmission of the disease, rotate away from hosts for 3 years, practice fall tillage to help reduce crop residue (and therefore inoculum), purchase seeds/seedlings from reputable companies, inspect seedlings regularly, monitor and be prepared to spray preventative fungicides.
Figure 1. Water soaked lesion where cotyledons attach to the hypocotyl, a symptom of gummy stem blight.
Anthracnose, caused by Colletotrichum orbiculare can be confused with gummy stem blight, although symptoms usually don’t become severe until the canopy closes. Both diseases affect all aboveground plant parts and seed can again be the source of initial inoculum. Watermelon, cucumber, and honeydew melon can experience serious losses when susceptible cultivars are grown while squash, cantaloupe, and pumpkin are less susceptible. In cucumber and honeydew melon, leaf lesions start as small water soaked areas, eventually becoming somewhat circular and brown with a yellow halo. In watermelon, the lesions are often smaller than in cucumber, darker brown, and irregular shaped. When seedborne, anthracnose symptoms appear as a wilt of cotyledons and water soaked lesions on the stem near the soil line (Figure 2), below where the lesions occur in gummy stem blight. The best options to avoid anthracnose are to start by choosing resistant varieties, purchasing disease free seeds, monitor and inspect seedlings, rotate away from cucurbits for 3 years, and practice fall tillage to remove residue.
Figure 2. Water soaked lesion at the soil line, a symptom of anthracnose.
Another fungal disease worth mentioning is Fusarium wilt, with symptoms in watermelon of wilted seedlings or damping off.
Bacterial fruit blotch caused by Acidovorax avenae subsp. citrulli, is caused by a bacteria as the name suggests. Initial symptoms are similar across cucurbit species but often more severe in watermelon, appearing as water soaked lesions on the underside of cotyledons (Figure 3). Lesions will turn necrotic extending along the leaf veins and in severe cases can cause damping off in seedlings. Lesions on mature leaves are reddish brown and elongated along the leaf veins, but they are easily confused with other diseases like gummy stem blight and anthracnose. Seedborne transmission of bacterial leaf blotch is the most important source of inoculum and conditions in the warm and humid greenhouses help the disease become established and spread among seedlings.
Figure 3. Water soaked lesion on the underside of cotyledons, an initial symptom of bacterial fruit blotch.
Seedling grow out assays of 10,000-30,000 seeds per lot are used to screen for bacterial fruit blotch infected seed lots and help reduce the risk of outbreaks. Sanitation efforts such as cleaning and disinfecting trays, using new soil, separating seed lots, keeping humidity low, and watering seedlings at midday so they have time to dry before the evening can also help reduce the risk of an outbreak. Destroy all trays with symptomatic plants and remove and isolate adjoining trays for observation and monitoring of disease symptoms. Remaining trays should be treated with labeled copper fungicides until they are transplanted.
Angular leaf spot is another bacterial disease and is caused by Pseudomonas syringae pv. lachrymans. There are resistant cucumber varieties available but angular leaf spot can occur in all cucurbit crops. Symptoms can look similar to bacterial fruit blotch, so proper identification is key. Lesions start as water soaked angular lesions on the underside of leaves before becoming brown or straw colored and surrounded by yellow halos. Similar to bacterial fruit blotch, it is important to start with disease free seed, as both diseases can infect fruit later in the year and directly impact marketable yield. Sanitation, crop rotation, and the ability to identify symptomatic plants are important ways to protect your crops before they go into the field.