Posts Tagged ‘gummy stem blight’

Gummy Stem Blight Found on Watermelon and Cantaloupe Transplants

Friday, June 12th, 2009

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Gummy stem blight was diagnosed on watermelon and cantaloupe transplants this week from Sussex County. Nancy found abundant fruiting bodies of the fungus and plenty of spores. The cotyledons were black and the fungus had infected the stems on some plants. If these transplants are set out, most of the infected plants will die but not before possibly producing spores that could infect nearby plants that might not be showing symptoms. Fungicide applications in the field would be strongly recommended if you suspected or had confirmed infection on transplants. Normally we would suggest a band application of chlorothalonil (Bravo) as a preventative but with the possibility of high disease pressure a combination of chlorothalonil and Pristine would be suggested. In situations where there might be suspected resistance issues with Pristine or reduced activity with Pristine, there are two other products that are labeled for gummy stem blight but are not in the recommendations book since we have seen little data in the region. A fall test in SC in 2008 conducted by Dr. Tony Keinath at Clemson University showed very good control of gummy stem blight under heavy disease pressure with Folicur and Switch. Both products were alternated with 2 pts of Bravo after two applications of either fungicide. This is only one test but the results were significant. With the weather being what it is, alternating Pristine with one of these products may be an effective alternative.

gummy stem blight on watermelon transplants Two gummy stem blight infected transplants and a healthy one on the right.

MELCAST Disease Forecasting for Watermelons

Friday, May 15th, 2009

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

The weather based forecasting program MELCAST on watermelon will begin in the next week. MELCAST is a weather-based spray scheduling program for anthracnose and gummy stem blight of watermelon. If you received a report in 2008, you should automatically receive the first report next week. If your email or fax number has changed, please call us. If you do not receive a report and would like to, please call Jeri Cook at (410) 742-8788 and give us your name and fax number or e-mail address. MELCAST also is available online – bookmark the site http://mdvegdisease.umd.edu/forecasting/index.cfm. Click on the watermelon picture.

To use MELCAST for watermelons, apply the first fungicide spray when the watermelon vines meet within the row. Additional sprays should be applied using MELCAST. Accumulate EFI (environmental favorability index) values beginning the day after your first fungicide spray. Apply a fungicide spray when 30 EFI values have accumulated by the weather station nearest your fields. Add 2 points for each overhead irrigation that is applied. After a fungicide is applied, reset your counter to 0 and start over. If a spray has not been applied in 14 days, apply a fungicide, reset the counter to 0 and start over. Please call Kate Everts if you have any questions on how to use MELCAST on your crop (410) 742-8789.

Because of widespread resistance to Quadris in our area, chlorothalonil (Bravo, etc.) or Pristine plus chlorothalonil alternated with chlorothalonil is recommended when spraying according to MELCAST. If a serious disease outbreak occurs in your field, return to a weekly spray schedule.

MELCAST for Watermelons

Friday, May 30th, 2008

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

The weather based forecasting program MELCAST has begun for 2008. MELCAST is a weather-based spray scheduling program for anthracnose and gummy stem blight of watermelon. If you received a report in 2007 by fax, you should have received the first 2008 report last week. We are still working on the email addresses, but look for your email report today (Friday, May 30). If you are not receiving reports and would like to, please call Jeri Cook at (410) 742-8788 and give us your name and fax number or e-mail address. We have “migrated” the information to a new website, so bookmark the new site at http://mdvegdisease.umd.edu/. Click on the watermelon picture. (We are still in the process of moving the MELCAST-cantaloupe and the Tomcast reports.)

To use MELCAST for watermelons, apply the first fungicide spray when the watermelon vines meet within the row. Additional sprays should be applied using MELCAST. Accumulate EFI (environmental favorability index) values beginning the day after your first fungicide spray. Apply a fungicide spray when 30 EFI values have accumulated by the weather station nearest your fields. Add 2 points for each overhead irrigation applied. After a fungicide is applied, reset your counter to 0 and start over. If a spray has not been applied in 14 days, apply a fungicide, reset the counter to 0 and start over. Please call Kate Everts if you have any questions on how to use MELCAST on your crop : (410) 742-8789.

Because of widespread resistance to Quadris in our area, chlorothalonil (Bravo, etc.) or Pristine plus chlorothalonil alternated with chlorothalonil is recommended when spraying according to MELCAST. If a serious disease outbreak occurs in your field, return to a weekly spray schedule.

Watermelon and Cantaloupe Disease Update

Friday, May 23rd, 2008

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

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; keverts@umd.edu

 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.