VOLUME 6. No. 2 February 25, 1998

Maverick County Vegetable-Pecan Conference

The 1998 Vegetable-Pecan Conference will be held Thursday March 12 in the Maverick County show barn at 611 south Monroe in Eagle Pass. The conference will begin with registration and introductions at 8:00 a.m.

This year's conference will feature Larry Stein, Mark Black, Noel Troxclair and Lynn Brandenberger from the Texas Agricultural Extension Service and Roger Magee from the Texas Department of Agriculture. Topics for the conference will include results from 1997 spring cantaloupe trials, pecan orchard thinning and salinity management, diseases of pecan and their control, integrated pest management in vegetables and pecan orchards, pesticide laws and regulations and micro-irrigation of crops.

For more information on this year's conference contact Mr. Omar Montemayor, Maverick County Agricultural Agent at (830) 773-5064.

Quadris Receives Crisis Exemption for Cucurbits

The Texas Department of Agriculture has announced that Quadris (azoxystrobin) has been issued a crisis exemption for use on cucurbits for the control of gummy stem blight (Didymella bryoniae) and powdery mildew (Sphaerotheca fuliginea, and/or Erysiphe cichoracearum). Quadris fungicide, manufactured by Zeneca Ag Products, may be applied starting February 13, 1998 according to the crisis exemption. This fungicide may be applied only by ground equipment and a resistance management scheme must be followed to prevent a buildup of resistant fungi.

For more information regarding this crisis exemption contact the Texas Department of Agriculture at 1-512-463-7476

Guidelines for Management of Gummy Stem Blight with Fungicides by: Tom Isakeit

Quadris is a new systemic fungicide that has excellent activity against several fungal diseases of cucurbits. The Texas Department of Agriculture recently issued an emergency clearance for its use in Texas to control gummy stem blight, caused by the fungus, Didymella bryoniae. Under this clearance, Quadris is meant to replace Benlate, another systemic fungicide. Benlate is no longer effective for gummy stem blight control in the Lower Rio Grande Valley (and possibly, other growing areas) because of wide-spread resistance of populations of the pathogen to it. Quadris is not meant to replace protective, non-systemic fungicides such as chlorothalonil (Bravo), mancozeb (Dithane), and coppers (e.g. Kocide).

Rather, these fungicides should be used in a complementary fashion with Quadris, in order to maximize disease control, obtain the most economical plant protection, and prevent the development of Quadris-resistant pathogens.

Quadris must not be used to cure a gummy stem blight epidemic. To be effective, it must be applied before the onset of disease symptoms.

The frequency of Quadris use will depend upon disease pressure, or, more particularly, a

anticipated disease pressure. Since we don't know what weather to expect in the next few weeks, a grower's previous history with gummy stem blight in the spring, 1997 season will have a major influence on decisions to use Quadris. At this time, we do not have guidelines for Quadris use under all possible disease scenarios. We do know from Dr. Marvin Miller's fall, 1997 cantaloupe fungicide trial that Quadris is very effective for gummy stem blight control when there is a large amount of the fungus present and there are disease-favorable weather conditions. However, we don't have information on effective fungicides under conditions that are less favorable for disease development. Furthermore, the fall, 1997 trial targeted early-season control of gummy stem blight of cantaloupe, whereas, in other growing areas, or with watermelons, mid- to late-season development of epidemics are more important. Under these conditions, other fungicides should be just as effective as Quadris for disease control. These other fungicides include protective fungicides and new, experimental systemic fungicides.

Cantaloupe Disease Scenarios: Low vs. High Risk

Gummy stem blight is not a new disease to the Lower Rio Grande Valley. Traditionally, this disease appears on the crowns and stems of young cantaloupe plants early in the season, usually during a period of cool, wet weather. It has been "conventional wisdom" in Valley cantaloupe production that a directed stem drench of benomyl is the treatment that stops the development of this fungus. It is more likely that as the season progresses, warming temperatures play a major factor in diminishing disease development. There have been no studies of gummy stem blight epidemics in the Valley during the past 60 years and there have been no studies where anyone has demonstrated the effectiveness of benomyl drenches under Valley conditions.

The gummy stem blight epidemic experienced in the spring of 1997 was propelled by extended periods of wet, cool conditions through much of the growing season that allowed normally-insignificant gummy stem blight lesions on stems to progress onto vines and leaves and finally onto the fruit. If we do not experience the same sort of weather that we had last year, we will not have a severe gummy stem blight problem. Since we can not forecast the weather, there should be an application of Quadris on a preventative basis under conditions where the risk for gummy stem blight is greatest.

High risk for gummy stem blight: A grower has a high risk for gummy stem blight if the cantaloupe field has a previous history of gummy stem blight or if the field was previously cropped to cantaloupe in the past year or the year before. In such situations, the fungus is probably present in the field and an epidemic may develop if the weather conditions are favorable. With such fields, Quadris should be applied when 2 to 4 true leaves are present. In 7-10 days, a protective fungicide such as Bravo or Dithane should be alternated with Quadris. Two to three weeks after the first application of Quadris, it should be applied a second time if weather conditions have continued to be favorable for gummy stem blight development. If the weather has not been favorable for gummy stem blight, then a protective fungicide may be applied at this time. During this period, the grower should inspect the field for evidence of gummy stem blight development. If there is no evidence of gummy stem blight and if the weather is relatively dry, then protective fungicide sprays should be applied with the aim of managing other diseases such as Alternaria leaf blight and downy mildew, which are more typical foliar disease problems in Valley cantaloupes.

If the weather remains favorable for gummy stem blight, or, in other words, if we are having a repeat of spring, 1997 weather, one additional application of Quadris (i.e. a third one) should be applied, 7 to 14 days after the previous application of the protective fungicide.

Low risk for gummy stem blight: Fields that are at low risk for having gummy stem blight problems are those that are planted later in the season. In the spring of 1997, the gummy stem blight epidemic was more severe with early-planted cantaloupe than late-planted cantaloupe. Fields that have been out of cantaloupes during the previous two years are also at low risk. In low risk fields, Quadris may not be needed at all. A protective fungicide program should be sufficient, particularly if weather conditions are not favorable for disease development during the early part of the season.

Watermelon Disease Scenarios: Practically All Low Risk In south Texas, gummy stem blight has not been a problem on watermelon, even in the spring of 1997. Protective fungicides should be sufficient. The only exception is if plants are known to be exposed to the gummy stem blight fungus, for example, if there is an outbreak of gummy stem blight during the production of transplants. Even if transplants with visible lesions are culled, some adjacent, symptomless plants may have infections. Under favorable weather conditions, the infections may develop into stem lesions. In this case, one early application of Quadris is sufficient; subsequent fungicide applications should be with protective chemicals.

I have planted watermelon transplants on the Weslaco station to observe disease progress during this growing season. The transplants included those with obvious gummy stem blight symptoms, as well as symptomless plants from the same flat. The transplants were exposed to the cool, rainy weather during the weekend of Feb. 14-15. After one week, there has been no or a slight increase in the size of leaf lesions, although lesions on the petioles of infected plants did expand to the main stem. The application of Bravo did not prevent this expansion. However, new leaves of plants remain symptomless, including those plants that did not receive any fungicides.

The Use of Protective Fungicides

In the fall, 1997 fungicide trial, applications of Bravo controlled gummy stem blight, but not as well as Quadris. The key to an effective application of Bravo is good coverage by using a high volume of water (>30 gal/acre). In the same trial, Dithane was not as effective as Bravo. In growth chamber tests using seedlings, Dithane was as effective as Bravo for suppressing gummy stem blight. The Rohm and Haas Company recommends that for gummy stem blight control, Dithane should be applied at the maximum label rate, on a 7-day schedule. Additionally, they recommend an early application (i.e. at the two leaf stage), the use of a good surfactant, and good coverage by using an adequate volume of water. (These recommendations are also valid for other fungicides).

Specifics on Quadris Application

Quadris is to be applied at the rate of 11 to 15.4 fluid ounces (0.18 to 0.25 lb. a.i.) per acre per application. This rate is on a broadcast basis. If the material is applied as a directed band, only about one-third of the above amounts will be applied, resulting in a more economical application. There is a maximum of four applications per season (64 fl. oz). Quadris is to be applied by ground equipment only, but not by drip irrigation. It is not cleared for drip application, nor is there any evidence that it will work in this manner. Quadris should not be applied sequentially, it should be alternated with protective fungicides. If Quadris is used sparingly, and in conjunction with other fungicides, it will not be lost from the fungicide arsenal because of a buildup of pathogen resistance.

Use and Disposal of Plastics in Agriculture

A report detailing the use and disposal of agricultural plastics has been prepared by Amidon Recycling. This technical report from the American Plastics Council will be released this fall and provides a good overall look at how plastics are utilized in our industry and what becomes of them after their use.

In the interim, there are a few copies of the 1994 report available. If you are interested in receiving the 1994 report call the American Plastics Council at: 1-800-243-5790 or call Arthur Amidon at (603) 654-6577.

Spring 1997 Cantaloupe Trial Results Available by: Omar Montemayor & Lynn Brandenberger

Results from the last spring's cantaloupe trial in Maverick County are available by contacting the Maverick County Extension office at (830) 773-5064 or on the World Wide Web at http://extension-horticulture.tamu.edu/southtex.

In the trial there were differences between varieties for earliness, yield and sweetness, (table 1). The trial included cantaloupe varieties that were classed as either early or main season types. Of the ten varieties included in the trial, four early types including; Impac, Pacstart, Valleypac and Primo were considerably earlier than the main season types in the trial. The majority of high yielding varieties in the trial produced a higher percentage (64-96%) of size 6 and 9 melons. Cloudy wet weather during this past spring may account for these larger melons and also for the lower levels of sweetness recorded in the trial.

Lynn Brandenberger/Editor

The information given herein is for educational purposes only. References to commercial products or trade names are made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.

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