In This Issue Lower Rio Grande Irrigation Conference Microbial Safety of Produce Meeting Diagnosis and Control of Foliar Diseases of Watermelon in South Texas It's That Time of Year! |
Lower Rio Grande Irrigation Conference
The 1997 Irrigation Conference for the Lower Rio Grande area will be held on Thursday December 4 at the Harlingen Community Center (201 East Madison in Harlingen) beginning with a breakfast at 7:00 am. The conference is a cooperative effort of the LRGV Irrigation Districts Managers' Association, Harlingen Parks and Recreation and the Texas Agricultural Extension Service. Registration is payable at the door and includes breakfast, lunch and admission to the trade show and all technical sessions.
Session topics will include the following; Current water supply and projections for 1998, Mexican dams and management of the Rio Grande, Update on the Rio Grande integrated water resources plan, Report from irrigation district managers, Il Nino and drought, District water metering, On farm water metering, Irrigation innovations and Crop production under limited water supply. Simultaneous Spanish language translation of all sessions will be provided by Arturo Mijares of Monterrey, Mexico.
If you would like more information regarding the conference contact either Brad Cowan (956) 383-1026, Terry Lockamy (956) 399-7757 or Guy Fipps at (409) 845-7454.
Microbial Safety of Produce Meeting
The Food and Drug Administration is announcing a series of meetings on the microbial safety of produce. The meetings are intended to give an overview of and obtain input on the general draft guide titled: Guide to minimizing microbial food safety hazards for fresh fruit and vegetables. The meeting for Texas will be held on December 8, 1997 near San Antonio at the Helotes 4-H center located at 12132 Leslie Road, Helotes, TX 78023. For further information about the meeting contact Camille E. Brewer, Food Safety Initiative, FDA, Rm 3169, 200 C Street, SW., Washington, DC, (202) 260-8920. Written comments on the meetings may be submitted by December 19 to the Dockets management Branch (HFA-305), Food and Drug Administration, rm. 1-23, 12420 Parklawn Dr., Rockville, MD 20857.
Diagnosis and Control of Foliar Diseases of Watermelon in South Texas: Tom Isakeit
In this article, "foliar disease" will be limited to a symptom that not only appears on the foliage, but started there. Thus, diseases where leaves are wilted because of root rot or clogging of the vascular system will not be discussed. In diagnosing diseases, it is important to examine all parts of the plant, including ones far removed from the obvious symptom. There are many foliar diseases caused by microorganisms, but this article will only treat the major ones in south Texas.
Cercospora leaf spot, caused by the fungus, Cercospora citrullina, is the most prevalent foliar disease of watermelon in south Texas, but it is not usually a serious problem. It has a characteristic symptom: circular brown spots with white or tan centers. Sometimes the area surrounding the brown spot is slightly chlorotic. Heavy infection leads to defoliation and therefore chemical control is advisable. The disease is favored by wet conditions and warm temperatures of 80-90F. This pathogen overwinters on crop debris and cucurbit weeds, yet I have seen the disease on new watermelon land and have noticed only healthy-looking citron melons and burgherkins in the midst of symptomatic watermelon plants. Airborne movement of spores is important for introducing the pathogen into fields. The disease can be managed with a spray program of protectant fungicides such as coppers (e.g. Kocide), mancozeb (Dithane, Penncozeb). maneb, and chlorothalonil (Bravo).
Downy mildew, caused by the fungus Pseudoperonospora cubensis, does not reliably appear in fields, but its occurrence should always be a cause for concern because of its ability to quickly increase and become uncontrollable. It is first seen as irregular, yellow areas on the upper leaf surface. The centers of the areas become brown and they are surrounded by a yellow area. Under conditions of high humidity, fuzzy, grayish areas are seen on the lower leaf surface. It is best to make the examination for this early in the morning. Alternatively, suspect leaves can be removed and kept several hours in a plastic bag with a moist paper towel to provide high humidity. As the disease progresses, the entire leaf dies and the foliage in a field where this disease is running rampant appears as if it were burned. The disease is favored by cool (60F night and up to 77F day) temperatures along with rain or continuous high humidity. This disease can occur in a field miles away from other cucurbits, indicating that the spores really get around. Excellent control of this disease can be achieved with mefanoxam (Ridomil Gold), which must always be used simultaneously with a protectant fungicide such as mancozeb or chlorothalonil to prevent the emergence of resistant strains. Furthermore, these sprays must be applied on a preventative basis, before the disease becomes established in a field. Some growers report that they only use chlorothalonil or mancozeb to effectively manage downy mildew, but, again, the key to their effectiveness is regular, preventative applications.
Alternaria leaf blight, caused by the fungus Alternaria cucumerina, is not as much a problem on watermelon as it is on cantaloupe. The symptom is a large, dead lesion on the leaf. This symptom could easily be confused with similar symptoms produced by insects or physical damage. Furthermore, leaf tissue killed by other things can be colonized by other, saprophytic Alternaria species. The disease is favored by continuous wet conditions, but it can be controlled by the same fungicides that control Cercospora leaf spot.
Gummy stem blight, caused by the fungus Didymella bryoniae, causes large, circular dark brown spots on leaves. Later, small black, pimple-like structures appear in these spots. This is an infrequently seen, minor disease of watermelons in south Texas and it's a good thing, too, because the fungicide of choice, benomyl, can not be relied upon in this area. This is an important disease of watermelon in the northern part of the state and in other states, just like another not-seen-in-south-Texas fungal disease known as anthracnose. Evidently, south Texas is not humid enough for these diseases to thrive on foliage.
Powdery mildew, seen as a white, powdery residue primarily on the upper leaf surface, is caused by two species of fungi, Sphaerotheca fuliginea and Erysiphe cichoracearum. The former species is prevalent in south Texas (the two species can be differentiated by microscopic examination). This disease is favored by high humidity, but not free water on leaves. It tends to occur on older leaves and later in the growing season, so control is not always warranted. Two systemic fungicides, Benlate (benomyl) and Bayleton (triadimefon), are no longer effective because of resistance. There are two types of protectant fungicides labeled for powdery mildew, namely, coppers (e.g. Kocide) and sulfur (e.g. Copper/Sulfur F). These fungicides are not very effective under south Texas conditions.
The bacterial pathogen that causes fruit blotch, Acidovorax avenae subsp. citrulli, can also cause foliar symptoms. Curiously, where I have seen abundant fruit blotch in south Texas fields, I have seen no foliar symptoms on the affected plants. Yet, the only time I've seen abundant leaf symptoms of this pathogen, there were no fruit symptoms. Other watermelon growing areas report both leaf and fruit symptoms, so it is likely related to weather conditions (although the cultivar or other factors may affect symptom development). The pathogen can cause large, circular brown lesions, usually near the edge of the leaf, but this is not reliable for diagnosis. The unique symptom are the irregular, rust-colored lesions that run along leaf ribs. On the underside of the leaf, the leaf rib is water-soaked. Leaf lesions can serve as a source of bacteria that can infect fruit, but I suspect that in most cases, by the time symptoms are seen on the leaves, it is too late to prevent fruit infection. This is why it is prudent to use copper sprays on a preventative basis.
With viruses, there is usually no necrosis (death) of leaf tissue. The squash leaf curl virus, transmitted by whiteflies, will cause an irregular yellowing of leaves and this is always associated with distortion of leaf shape. This is a common disease of fall-grown watermelons and large yield losses can be avoided by using transplants, or direct-seeding when there are low local populations of whiteflies. A green mosaic symptom (i.e. light and dark areas of green), sometimes with leaf deformation, can be caused by two aphid-transmitted viruses, the watermelon mosaic virus and papaya ringspot virus W. These viruses are infrequent on south Texas watermelons, usually limited to one or two plants in a field and never spreading. These viruses can cause major losses in other production areas. The tobacco ringspot virus, which is transmitted by the dagger nematode, causes small, chlorotic or necrotic spots, or ringspots on leaves. It is not an important disease, except when it produces fruit symptoms, for example, necrotic pimples, or raised, necrotic "target spots". It can be a problem in weedy fields or land that was recently brought into cultivation. Cultivation and weed control can reduce this disease because they cause reductions of the nematode populations in soil.
This article only dealt with infectious diseases. Other, non-living agents can cause foliar symptoms and these can be more difficult to diagnose. However, the first step to diagnosing these problems is to rule out the infectious pathogens.
It's That Time of Year: Lynn Brandenberger
Well, it's that time of year again. I know that you're wondering what I'm talking about, so I'll tell you, it's Thanksgiving. Thanksgiving is one of those uniquely American holidays that we celebrate. Just mention the word and most of our minds immediately begin to reminisce about past Thanksgivings including mountains of food, hours of parades and cheering for our favorite football team. But wait there's more, much more than just feeding our faces and enjoying the company of friends and family.
This tradition that we celebrate each year goes back quite a ways. Each of us remembers what we learned as children about the pilgrims and all their struggles to survive and how advice from their Indian neighbors and assistance from the Almighty provided for their needs. But giving thanks goes back even farther, probably to the beginning of mankind, at least to biblical times when feasts were scheduled to celebrate and give thanks for the harvest.
So what am I thankful for? Well of course, I'm thankful for the bounty that American agriculture provides for us. Specifically, I'm thankful that there are farmers and their families dedicated enough to take the risks and make the sacrifices necessary to produce that bounty. Much of the general public isn't aware of the risks inherent in producing the food and fiber that we have come to take for granted. They assume that regardless of what happens it will be there, on time and in abundance and at a very reasonable price. They don't understand that although farmers may do every thing in their power to bring a crop to market, mother nature may and often does throw a wrench into the works. I'm also thankful that farmers have the courage to go on after mother nature pulls those mean stunts on us. It takes tremendous faith, optimism and fortitude to go forward after a major crop disaster such as we had this spring. Most of all I'm thankful that we have the resources that are provided on this globe that make agriculture possible. The soil, air, water, minerals, plant, animals and the human resources necessary for their stewardship in order to provide the bounty that we depend upon for life.
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.