Posts Tagged ‘watermelon fruit blotch’

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.

Preventing Spread of Bacterial Fruit Blotch in Watermelon Transplants

Thursday, April 12th, 2012

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

Now that transplant production is in full swing, it is timely to review what we do and don’t know, about bacterial fruit blotch (BFB) of watermelon. BFB of watermelon is caused by the bacterium Acidovorax avenae subsp. citrulli. Warm, humid conditions in greenhouse transplant houses are highly favorable for the spread of disease and the development of BFB symptoms. Although the disease spreads quickly in the transplant houses, it often is not noticeable in the field until shortly before harvest. There, BFB is damaging because it causes 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. In the image below, the leaves were incubated and the lesions have progressed along the veins and are obvious. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. The pathogen also causes disease on muskmelon or cantaloupe, honeydew, and on squash and pumpkin.

There are many steps that can lower the risk of development and spread of BFB on watermelon in the transplant house. All seed in a commercial greenhouse should have been tested and found to have “no evidence” of the pathogen. Don’t grow experimental lots that were not tested in a commercial house. Remember too, that testing, and “no evidence” does not guarantee that BFB will not develop, it is one of many steps to reduce the risk of disease. Inspect seedlings beginning at cotyledon expansion and at frequent intervals afterward. If BFB is suspected, send plants to the University of Maryland or University of Delaware diagnostic lab, or the Lower Eastern Shore Research and Education Center, for identification. Destroy all trays with symptomatic plants and those within a five foot radius. Remove adjoining trays to a separate – isolated – area for observation. Monitor these isolated seedlings daily and destroy trays where symptoms develop. All plants in the greenhouse should be sprayed with copper such as Kocide or Nordox. The applications should continue until the plants are shipped or transplanted to the field.

All the greenhouse surfaces should be sterilized prior to the production cycle. A solution of one part of bleach to nine parts of water, or Greenshield or Physan can be used on implements or benches. Don’t reuse trays.

Additional good practices for greenhouse transplant producers are:
· Workers should wash their hands and use a shoe bath when entering the greenhouse to work.
· Minimize the number of people that enter the greenhouse.
· Eliminate all weeds in and around the house.
· Maintain low humidity in the greenhouse.
· Water plants at their base and avoid splash between plants.
· Keep greenhouse flaps closed if it is windy.
· Segregate seedlots and separate them from each other with a vertical plastic sheet to avoid spread by splash or in aerosols.

Bacterial fruit blotch symptoms after incubation. Note the range of symptoms from small lesions on the true leaves to advanced lesions on the cotyledons.

Bacterial Fruit Blotch Epidemiology

Friday, May 20th, 2011

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

I have continued to receive questions about bacterial fruit blotch (BFB), including how it spreads through a transplant house and the production field during the growing season. To understand BFB it is important to understand a little about its’ epidemiology.

Infected transplants are the most common source of BFB inoculum in Delmarva fields. (However, it can overwinter on debris and on infected volunteer plants). The reason that transplants remain a major source of inoculum is that 100% detection of infested seed is not possible. Many steps are taken by seed companies and transplant growers to avoid infestation, detect infection, and eliminate the disease. However, currently we don’t have the technology to accomplish this.

The environmental conditions in watermelon transplant production houses are highly conducive to disease development and spread of BFB. High temperatures, high humidity, overhead irrigation and high plant populations favor BFB and result in rapid symptom development. As a result, detection of BFB in transplant production is common.

BFB spreads from plant to plant on hands or equipment, in splashing water (irrigation or rain), or in aerosols. Once it lands on a plant it enters (infects) through wounds or stomates.

In commercial fields, spread of BFB will occur most rapidly under warm, humid conditions and during rainfall or overhead irrigation. When the bacterium is deposited on the watermelon flower, it can penetrate through stomates and infect fruit. The infections that cause fruit loss can only take place during flowering and fruit development before wax deposition (wax seals the stomates). That means that the yield damaging infections occur only during flowering and for about 3 weeks afterward. Although infections occur early in the season, fruit symptoms often do not develop until harvest. Chemical treatments (i.e. copper) to protect the crop should be applied before and during flowering, and for three weeks afterward.

Low humidity in watermelon fields prevents the development of both foliar and fruit symptoms. In fact infested seed can be produced from completely symptomless plants. Simply put, infected plants can appear symptomless. (This is one reason why, during seed production, infections cannot be completely eliminated based on symptoms).

 

Bacterial Fruit Blotch Continued…

Friday, May 6th, 2011

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

As of Thursday, May 5, no Bacterial Fruit Blotch (BFB) had been confirmed in Delaware or Maryland. Please continue to monitor both locally grown transplants and transplants purchased from other areas, because BFB has been reported in Georgia.

Management of BFB in greenhouse plant production includes the following steps: 1) use seed that has been tested for Acidivorax avenae subsp. citrulli, 2) monitor plants for BFB symptoms, 3) if potential symptoms are observed, submit plants for diagnosis. If BFB is confirmed in a greenhouse, the symptomatic transplants and those in a 15-foot radius should be destroyed. Additional trays that are 15 to 20 feet from the infected plants should be removed and isolated in a warm humid location and observed closely for five days for symptom development. If symptoms develop, then the epidemic has not been contained and additional plants should be destroyed.

If, despite best practices, BFB is observed in a field following setting out transplants, BFB will continue to spread. The rate of spread depends on the environment and irrigation practices. Spread will be fastest in fields irrigated with a travelling gun, intermediate where center pivot irrigation is used, and slowest with drip irrigation. Likewise, spread will occur within a field during rainfall, especially during “driving” rains. Spread of BFB from field to field in air is not common (though the bacterium could move in an aerosol). However, spread from field to field will occur on tractors or truck tires, cultivation equipment, peoples’ hands and shoes, and other direct contact.

The best spray practices to minimize BFB spread in the field are to use copper and Actigard. Copper fungicide should be applied weekly beginning before flowering until after fruit set (approximately the first five sprays). Another option is to include a copper fungicide in the first, third and fifth fungicide application and include Actigard or Actigard plus copper in the second and fourth fungicide application. These programs have provided suppression (but not elimination) of BFB. Additional labor at harvest may be necessary to separate symptomatic fruit from symptomless fruit.

 

Bacterial Fruit Blotch Detected in GA Watermelon Seedlings

Friday, April 29th, 2011

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

I just received word from Dr. David Langston in Georgia that bacterial fruit blotch (BFB) was confirmed on seedlings destined for shipment to other watermelon producing areas. Some plants may have been shipped before the outbreak was identified. At this time we do not know if any seedlings were shipped to Maryland or Delaware. However, increased scouting of transplants is warranted. Because BFB is seed-transmitted, locally grown transplants should also be examined.

BFB of watermelon is caused by the bacterium Acidovorax avenae subsp. citrulli. The disease is damaging because it causes large olive green to brown water-soaked lesions on fruit (Figure 1), making them unmarketable. Symptoms of BFB on seedlings are water-soaked areas of the lower surface of the cotyledons and inconspicuous lesions on leaves (Figure 2). BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die.

Conditions in greenhouse transplant houses are highly favorable for the development of BFB symptoms and the spread of disease. If BFB is suspected, please send plants to a diagnostic lab (University of Maryland or University of Delaware) for identification. In the meantime, 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. After symptomatic plants and adjoining trays are discarded, spray the remaining trays with a labeled fungicide and continue applications until the plants are shipped or transplanted to the field.

Figure 1. Olive green water-soaked lesion on watermelon fruit. (Image courtesy David B. Langston, University of Georgia, Bugwood.org)

Figure 2. An inconspicuous lesion of bacterial fruit blotch on a watermelon transplant.

 

Watermelon Fruit Blotch

Friday, August 13th, 2010

Nancy Gregory, Plant Diagnostician; ngregory@udel.edu and Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Bacterial fruit blotch on watermelon has shown up in Delaware for the first time this year. This disease is caused by the bacterium Acidovorax avenae subsp. citrulli, which is most commonly seed-borne. Fruit blotch is favored by warm, humid conditions. The typical symptoms include a dark olive green irregular stain on the fruit. It appears water-soaked, but can be dry. The blotch will spread in size, but usually doesn’t extend down into the flesh of the fruit. Older lesions may split open, and then fruit rot can occur from the entry of other bacteria and fungi. Once fruit matures, the waxy rind prevents infection, so infections seen now probably occurred at fruit set or in early fruit development. Fields ready for harvest should be kept dry, and severely affected fruit or those with splits or wounds to the rind should be culled. Contaminated seed or seedlings are the primary cause of infection, but the bacteria can survive on crop debris and weed hosts. On Delmarva, we have observed instances where the pathogen overwintered and survived on watermelon “volunteer” plants grown in alternate, non-host years. Fields with known infections should be rotated away from cucurbits, plowed, and weeds and volunteer watermelon plants controlled. Greenhouse sanitation and clean seedlings for the next year are important. Copper or Tanos (8.0 – 10.0 oz/A) applied every seven days will suppress disease progress.

Watermelon fruit blotch