Posts Tagged ‘angular leaf spot’

Inspect Watermelon and Cantaloupe Transplants, New Bacterial Fruit Blotch Factsheet

Thursday, May 3rd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

As watermelon and cantaloupe transplanting continues on Delmarva, growers are reminded to inspect plants before they are transplanted into the field for signs of disease including Bacterial Fruit Blotch, Gummy Stem Blight, and Angular Leaf Spot.

Kate Everts and Gordon Johnson have put together a new factsheet on Bacterial Fruit Blotch, which will be of interest to watermelon growers. It is available online here:

Watermelon and Cantaloupe Disease Update

Friday, May 23rd, 2008

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

The cool nighttime temperatures and overcast and rainy days are holding back growth of watermelon and cantaloupe transplants that have been planted in the field. Although I’ve received several watermelon transplants for disease diagnosis, most of the damage on the samples is weather related – not disease. However, continue to carefully examine transplant lots going to the field.

Gummy stem blight remains the most common seedling disease in our production region. Look for infected necrotic cotyledons and water-soaked lesions extending from the cotyledons into the stem. As the disease advances, stem lesions will become tan and small pycnidia can be seen with a hand lens.

Gummy Stem Blight

Gummy stem blight in tray. Note the necrotic cotyledons, tan lesion on stem and the tiny black spots, which are the fruiting bodies of the pathogen.

I have seen one case of suspect angular leaf spot (ALS) on cantaloupe this spring. This bacterial disease occurred in Delmarva’s greenhouses in 2003. Symptoms are small dark brown irregular lesions on cotyledons or leaves. Lesions may have a chlorotic halo and may appear “shiny” (due to bacteria on the lesion surface). ALS is favored by cool, wet weather. 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. Past experience with the strains that have appeared here on Delmarva is that conditions after transplanting to the field do not favor ALS disease development. However, it is important to have the disease identified. If ALS is confirmed in the field applications of fixed copper plus mancozeb will minimize spread of disease. Also, avoid working fields when foliage is wet.

angular leaf spot

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

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 3rd, 2008

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

 The fungal diseases gummy stem blight, Alternaria leaf blight, and anthracnose can be introduced into the greenhouse on watermelon seed or through inoculum from a previous crop. To minimize the occurrence of these diseases, the greenhouse should be disinfected before planting (benches, walls, walkways, etc.). The seed source should have tested negative for the pathogen with a minimum assay number of 1,000 seeds. Use clean transplant trays (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. Keep relative humidity as low as possible through proper watering and good air circulation in the greenhouse.

 As the seedlings develop, inspect them carefully. Infected seedlings will have small brown lesions on the leaves and/or water-soaked lesions on the stem. Diseases that are transmitted on seed often are randomly located throughout the greenhouse. Initial infections will occur as ‘foci’ or clusters of diseased plants. 

gummy stem blight

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

If the seedlings appear diseased, destroy all trays with symptomatic plants. Remove adjoining trays to a separate area for observation. Monitor these seedlings daily and destroy trays where symptoms develop. Do not ship any trays containing plants with disease symptoms. After symptomatic and adjoining trays are discarded, spray the remaining trays with a labeled fungicide and continue applications until plants are shipped.

 Bacterial fruit blotch (BFB) of watermelon is caused by a bacterium that also is seedborne. Initial symptoms of BFB are water-soaked areas on the lower surface of the cotyledons. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. Angular leaf spot, which also is a bacterial disease, occurred in Delmarva’s greenhouses several years ago. Symptoms are small, dark brown, irregular lesions on cotyledons or leaves. ALB is favored by cool wet weather. Usually conditions after transplanting to the field do not favor ALB disease development.

bacterial fruit blotch

Small inconspicuous lesion of bacterial fruit blotch

Fusarium wilt also can be seedborne. 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). No lesions are observed along the stem or petiole but the vascular system is discolored and tan, pink or brown. This disease is of special concern because new strains or races can be introduced into an area on seedlings grown from infested seed.