Posts Tagged ‘gummy stem blight’

Disease ID for Pumpkin

Friday, July 20th, 2012

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

I recently wrote an update article about fungicide programs for pumpkin http://agdev.anr.udel.edu/weeklycropupdate/?p=4429. However, because many fungicides are effective on some diseases, but not others, it is important to be able to identify the diseases present in a field as you design your spray program.

Foliar Diseases
The most common foliar diseases of pumpkin are powdery mildew, downy mildew, white speck (Plectosporium), gummy stem blight and anthracnose.

Powdery Mildew

Figure 1a. Powdery mildew sporulation covering leaves and defoliating pumpkin plants.

 Figure 1b. Close up image of a leaf showing the “powdery” white sporulation on the upper surface of the leaf. Note that sporulation is usually seen first on the lower leaf surface. Scout a field by looking at the under surface of 45 old leaves in a field each week. Begin targeted sprays for powdery mildew when it is first observed.

White Speck (Plectosporium)

Figure 2. White speck or Plectosporium on the leaf causes tan spindle shaped lesions which form on the veins and result in distorted leaves. Plectosporium also causes lesions on the fruit (Figure 5).

Downy Mildew

Figure 3. Downy mildew lesions are initially seen on the upper surface as angular water soaked or yellow spots (3a) that are limited by the leaf veins. The angular nature of the lesions is especially evident on the lower leaf surface where sporulation occurs (3b). Look for grey angular lesions on the under surface of leaves after dewy nights. Lesions become necrotic over time.

Anthracnose

 Figure 4. Anthracnose will initially be small tan lesions with darker margins (image courtesy of B. Precheur, Ohio State Univ.). They will expand as they age and damage large portions of the leaf. They may develop small holes in the leaf. Anthracnose also causes lesions on the fruit (Figure 7).

Fruit Diseases
There are several pathogens that cause fruit rot on pumpkin. To manage fruit rot the single most important practice is to follow a good fungicide management program in the field. The same fungi that cause white speck, black rot and anthracnose also cause lesions on the leaves. If the leaves are protected from disease, the fruit will be less likely to become diseased. In addition to protecting fruit from rot, a good spray program will protect “handles” from damage and will maintain foliage health and keep sunscald at a minimum.

White Speck (Plectosporium)

Figure 5. White speck (caused by Plectosporium, formerly Microdochium) causes white or tan “pimples” on the fruit.

Black Rot

Figure 6. Black rot (caused by Didymella bryoniae the same fungus that causes gummy stem blight on the foliage) results in large grey lesions on fruit.

Anthracnose Fruit Rot

Figure 7. Anthracnose fruit rot (caused by Colletotrichum spp.) appears as smaller grey lesions on fruit.

Fusarium Fruit Rot

Figure 8. Fusarium fruit rot (Fusarium solani) causes a relatively dry fruit rot that initially appears as small white or pink spots as in this photo. Eventually however, the lesions may become black or tan because of saprophytic growth.

Southern Blight

 Figure 9. Southern blight on pumpkin fruit (Sclerotinia rolfsii) appears as a fan shaped white growth embedded with small round brown “seeds”.

Phytophthora Blight


Figure 10. A young target shaped lesion (10a) of Phytophthora blight (caused by Phytophthora capsici). Large lesion where fruit was in contact with soil (10b). Close up image of P. capsisi fruit lesion with felt-like sporulation (10c).

MELCAST Fungicide Scheduling for Watermelon

Friday, May 18th, 2012

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

Gummy stem blight (caused by Didymella bryoniae) is the most important foliar disease of watermelon in Maryland and Delaware. The disease affects leaves (Fig. 1), stems and vines of watermelon, resulting in fewer and smaller fruit. Anthracnose (Fig. 2), which is caused by Colletotrichum orbiculare, also occurs yearly. In MD and DE, yield losses due to gummy stem blight and anthracnose of 20 to 100% would occur in the absence of effective fungicidal control.

Beginning in 1997 the DE IPM, MAR-DEL Watermelon growers, Maryland Vegetable growers, and other grant funds have supported dissemination of a weather-based fungicide application program for watermelons, Melcast. Melcast is a weather-based spray advisory program for watermelon developed at Purdue University. The program uses hours of leaf wetness and temperature during leaf wetness periods to determine when a fungicide should be applied. In MD and DE, we have slightly modified Melcast so that fungicides are scheduled earlier. As a result, fungicides scheduled by Melcast, successfully manage anthracnose and gummy stem blight. Growers that use Melcast report reducing their fungicide applications by two per season compared to standard schedules. Six research trials were conducted over three years in our region to evaluate Melcast. In four of those trials yield was the same when sprays were applied according to Melcast compared to weekly applications. In one trial yield was higher, and in one trial yield was lower, when sprayed according to Melcast in comparison to weekly sprays.

Since our original trials of Melcast, several newer and highly effective fungicides have been registered for gummy stem blight and anthracnose. We are testing Melcast again with these effective materials. To use Melcast on your farm, please call Karen Adams at (302)856-7303 and give us your name and Fax number or e-mail address. More details about how the program works are available at our Disease Forecasting Web page: http://mdvegdisease.umd.edu/forecasting/index.cfm

Figure 1. Large dark brown foliar lesions of gummy stem blight

Figure 2. Anthracnose lesions on cucurbits are angular in appearance. Tiny black spots can be seen through a hand lens in the tan centers of lesions.

MELCAST for Watermelons

Friday, May 11th, 2012

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

The weather based forecasting program MELCAST on watermelon will begin next week. MELCAST is a weather-based spray scheduling program for anthracnose and gummy stem blight of watermelon. If you received a report in 2011, 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 Karen Adams at (302)856-7303 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 if you have any questions on how to use MELCAST on your crop (Kate Everts at 410-742-8789).

Do not use Quadris, Cabrio or Flint on watermelons in Maryland or Delaware. Under low disease pressure, use Chlorothalonil (Bravo, etc.) applied according to MELCAST. Under high disease pressure alternate chlorothalonil with Pristine plus chlorothalonil, Folicur plus chlorothalonil, Inspire Super plus chlorothalonil or Luna Experience plus chlorothalonil applied according to MELCAST. If a severe disease outbreak occurs in your field, return to a weekly spray schedule.

Inspect Watermelon and Cantaloupe Transplants, New Bacterial Fruit Blotch Factsheet

Thursday, May 3rd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

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: http://agdev.anr.udel.edu/weeklycropupdate/wp-content/uploads/2012/05/BacterialFruitBlotchFactsheet.pdf

Understanding Seed Waivers and Seed Born Diseases

Thursday, April 26th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A number of vegetable diseases can be transmitted through infected seed. For this reason, seed companies have developed quality assurance programs based on testing a certain amount of seeds in each seed lot for specific diseases. This most often involves the seed company growing out plants from these test lots and having trained individuals inspect the plants for signs of the disease. If there are suspicious plants, they are then further tested in the laboratory to confirm the disease. Sometimes seeds are tested directly for the specific disease organism (bacteria, virus, fungus). Only seed lots that have no disease detected are sold. For watermelon and cantaloupes, seed lots are tested for bacterial fruit blotch and often for gummy stem blight.

Because of past liability issues, growers are required to sign waiver forms to purchase watermelon and cantaloupe seeds from most companies. While this is often thought by growers to be a routine annoyance to purchase seeds, it is important to read the waiver forms and understand their implications.

These waivers commonly spell out what diseases the company tests for. The waiver will often have information on the testing process for these diseases. There will also be information about the diseases that the grower should know and often there will be detailed descriptions of how the disease develops and how to identify the disease.

In all waivers, there will be an important statement emphasizing that that the grower accepts the risks associated with those diseases.

The waiver may also include information on risk of nonperformance, assumption of risk, disclaimers or limitation of warranties, limits of liability, limits on damages, how to file a claim, statute of limitations on claims, arbitration of seed disputes (required by some states), expected remedies, limit on sales or transfers of seed, and attorney’s fees.

Once a seed waiver is signed then the seed company is protected from liability and this will reduce the ability of a grower to receive compensation if a seed borne disease does appear.

All growers are encouraged to understand what seed borne diseases are common with the vegetable crops that they grow, whether or not seed is treated or tested to reduce the chance of disease occurring, how to identify specific seed borne diseases, and how to manage seed borne diseases if they do occur (in greenhouse transplants or field plantings).

Growers should also maintain close relationships with seed suppliers and contact them immediately if a seed borne disease is suspected.

Watermelon Seedling Diseases in the Greenhouse

Thursday, April 5th, 2012

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

Bacterial fruit blotch (BFB)
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 are water-soaked areas of 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 (disinfect trays if they will be reused) and new soil must be used. 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 fungicide and the applications continued until the plants are transplanted to the field.

Olive green water-soaked lesion on watermelon fruit. (Image courtesy David B. Langston, University of Georgia, Bugwood.org)

An inconspicuous lesion of bacterial fruit blotch on a watermelon transplant.

Other Greenhouse Diseases:

Angular leaf spot, which also is a bacterial disease, looks similar to BFB. This “look-alike” disease occurred in Delmarva’s greenhouses several years ago. Symptoms are small dark brown irregular lesions on cotyledons or leaves. Angular leaf spot is favored by cool wet weather. Usually conditions after transplanting to the field do not favor angular leaf spot disease development.

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.

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.

MELCAST for Watermelon

Friday, May 27th, 2011

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

The weather based forecasting program MELCAST on watermelon began on Wednesday (May 25). MELCAST is a weather-based spray scheduling program for anthracnose and gummy stem blight of watermelon. If you received a report in 2010, you should have automatically received the first report. If your email or Fax number has changed, please call us. If you did 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/. Click on the watermelon picture.

To use MELCAST for watermelons, determine which site is closest to your farm field. Apply the first fungicide spray when your 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 if you have any questions on how to use MELCAST on your watermelon crop (Kate Everts at 410-742-8789).

Because there is widespread resistance to strobiluron (group 11) fungicides in Maryland and Delaware, growers should alternate one of the following with chlorothalonil (Bravo, etc.); a tebuconazole product (such as Folicur), Inspire Super, or Switch. Resistance to Pristine has been recorded in many watermelon fields in the southern U.S. We have not found resistance to Pristine here in Delaware or Maryland, yet. However, Pristine should be used with great caution; always tank mixed with chlorothalonil; and alternated with a fungicide that has a different mode-of-action. If a serious disease outbreak occurs in your field, return to a weekly spray schedule.

 

Watermelon Gummy Stem Blight Fungicide Programs in 2010

Friday, June 25th, 2010

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

Our weather has not been highly conducive to gummy stem blight or anthracnose in the last two weeks. Therefore, under low disease pressure a good strategy is to apply Bravo on a 7-day schedule. Alternatively, our trials over many years have demonstrated that under low disease pressure the spray intervals can be lengthened. Following the weather forecaster ‘MELCAST’ http://mdvegdisease.umd.edu/forecasting/index.cfm can help determine the safe interval that can be used without the likelihood of risking disease increases.

There are several fungicides available for gummy stem blight management. Although several products are available, the usefulness of some of these products is limited by resistance development in the pathogen. On Delmarva we have confirmed the presence of resistance in Didymella bryoniae, the pathogen, to fungicides in the FRAC code group 11 (strobilurins, including Quadris and Cabrio) and FRAC code 3 (demethylation inhibitors or DMIs, including Topsin M). Resistance to Pristine exists in Georgia, and therefore Pristine is not recommended in that state. We have not yet detected resistance to Pristine here. However many of our transplants are grown in the south and it would not be surprising to find that resistance has been introduced here.

The following are fungicide programs that performed well in trials in Maryland and other areas in the southeast US in 2009:

● Switch 14 fl oz/A (1 day PHI; FRAC codes 9 and 12) alternated with Bravo

● Folicur 8 fl oz/A (7 day PHI; FRAC code 3) alternated with Bravo

● Inspire Super at 20 oz/A (7 day PHI) alternated with Bravo (Inspire Super is a new product that has two active ingredients. Although one component is in the FRAC code 3 group – Inspire Super performed very well in 2009.)

● Pristine 12.5–18.5 oz/A (0 day PHI; FRAC codes 11 and 7) alternated with Bravo (Pristine continues to perform very well in my trials -it usually ranks at the top, although it is not always significantly better than other products. However, because it performs very poorly in Georgia trials and because of the potential for resistance development, use caution and monitor disease levels carefully, if you choose to use Pristine.)

Watermelon is susceptible to other diseases as well. Scout for downy mildew, Phytophthora fruit rot and powdery mildew. The presence of these diseases will require additional fungicide applications with products with different modes of action.

MELCAST Forecasting for Bummy Stem Blight on Watermelon and Alternaria Leaf Blight on Cantaloupe

Friday, June 4th, 2010

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

Forecasts for fungicide applications in watermelon and cantaloupes for the 2010 season are set to begin on Friday, June 4. If you have not received forecasts in the past and would like to receive them, please call Mrs. Jeri Cook (410-742-8788) and give her your email address. Locations will remain the same as last year. Maryland forecasts are for Woodbine, Waldorf, Galestown and Hebron. Delaware forecasts are for Coverdale Crossroads, and both northeast and southwest of Laurel. To use MELCAST, select weather data for the site closest to your farm.

Instructions on use of MELCAST are available at http://mdvegdisease.umd.edu/forecasting/index.cfm.

Gummy Stem Blight on Watermelon

Friday, June 26th, 2009

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

Gummy stem blight on watermelon is widespread on Delmarva now in fields that have been repeatedly cropped to watermelons and where the disease has been introduced on plant material. Those of you who have followed MELCAST (the gummy stem blight disease forecaster for watermelon) know that there have been many days where weather was excellent for disease spread. If you want to sign up to receive MELCAST forecasts please call Mrs. Jeri Cook and request to get on the list (410) 742-8788. MELCAST is delivered by email, fax or online http://mdvegdisease.umd.edu/forecasting/index.cfm. The following are guidelines that can be used to manage gummy stem blight in watermelons.

Application Timing
Apply the first fungicide spray when the vines meet in the row (when the longest runner is about 1 ½ ft long). If disease was observed on transplants prior to planting, you may need to apply the first fungicide earlier. Subsequent applications should be scheduled according to MELCAST or on a weekly basis. Using MELCAST should reduce sprays but also insure that the fungicide gets applied when it is necessary.

Application Guidelines
The fungus that causes gummy stem blight needs moisture to infect. Therefore, apply fungicides before a rain. If the fungicide application has a reasonable chance of drying it should be applied before rainfall so there is a protective barrier on the plant when the pathogen germinates. Modern fungicides are formulated to stick to leaves during rain, so reapplication immediately after a rain is not usually necessary.

Apply an Effective Fungicide
Don’t waste money and time on fungicides applied to treat diseases for which they are not effective. Know if a fungicide is effective before you apply it. The cause of gummy stem blight, Didymella bryoniae, has developed resistance to the strobilurin fungicides (eg. Quadris) and to benzimidazoles (eg. Topsin M). Resistance to Pristine has been reported in Georgia. Because of the prevalence of southern grown transplants on the Delmarva, growers should watch for possible resistance to Pristine here.

Recommended Fungicides
Under low to moderate disease pressure apply chlorothalonil (Bravo at 2-3 pt 6F/A) every 7 days. Under high disease pressure, apply chlorothalonil alternated with Pristine plus chlorothalonil. Other fungicide programs that performed well on gummy stem blight in 2008 are Folicur applied weeks 1, 2, 4, 5, and 7, alternated with chlorothalonil applied weeks 3 and 6; and Switch applied weeks 1, 2, 4, 5, and 7, alternated with chlorothalonil applied weeks 3 and 6.