Powdery mildew in cucurbits

Powdery mildew is the most prevalent disease of cucurbits. Each year it occurs throughout the country and is becoming more troublesome. It affects cucumbers, gourds, melons, pumpkins, and both summer and winter squash grown in the field and greenhouse. Until several years ago, watermelons were typically spared, but now these, too, are affected. Powdery mildew starts as small lesions on the older leaves. The spots enlarge and coalesce, forming a white powdery mycelium that looks like talc. The fungus produces spores that are dispersed by the wind. Hot temperatures and high humidity promote powdery mildew’s occurrence. Intensive dew worsens it, but rainfall actually lessens its development. Severely infected leaves can get completely covered with the white mycelium. They turn yellow, then brown, wither and die. Premature leaf loss leads to sunburn and undesirable fruit color. Advanced foliar infection decreases fruit sugar content, which adversely affects flavor and storability. Melons especially suffer. In yellow squash and zucchini, fruit size, yield and quality diminish. In pumpkins, handles shrivel and rot. Severe defoliation can wipe out entire cucurbit crops. In cucurbits three fungal species cause the damage—Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), Golovinomyces cucurbitacearum (syn. Erysiphe cichoracearum auct. p.p. ) and Golovinomyces orontii (syn. Erysiphe cichoracearum auct. p.p.). According to University of Florida materials, the latter is more widespread. Different races in each species complicate management. The Agricultural Research Service of USDA recently identified a number of new virulent races.

Photo by David B. Langston, University of Georgia.
This powdery mildew on watermelon shows the characteristic white spots.
Photo by Bob Ferguson.
Cornell Plant Pathologist Dr. Margaret McGrath shares the results of her extensive study of cucurbit diseases with growers at the Great Lakes Fruit and Vegetable Expo.

Importance of IPM

In her presentation on managing powdery mildew leaf blights and other diseases of cucurbits at the 2007 Great Lakes Fruit and Vegetable Expo in Grand Rapids, Mich., Cornell Plant Pathologist Margaret T. McGrath stressed an integrated approach. She recommends integrated pest management (IPM) because these pathogens have demonstrated an ability to evolve, and thus overcome fungicides and resistant varieties. However, the potential for pathogens to simultaneously overcome both resistant varieties and fungicides is low. In other words, if the pathogen succeeds in attacking one management tool, for instance the fungicide, resistance inherent in the cucurbit variety will provide control. Even an integrated program, however, demands careful management.

Resistant varieties

The first step in management is planting resistant varieties. McGrath notes, “Powdery mildew-resistant varieties have become an important component of an integrated program for managing powdery mildew in most cucurbit crops. There are now commercially available resistant varieties for cucumber, cantaloupe, yellow summer squash, zucchini, winter squash and pumpkin with good horticultural characteristics.” Cornell lists resistant varieties and extensive information on pests at http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_PM_2008.html. More cucurbits are bred today to resist powdery mildew. Commercial seed companies can provide information on resistance for their products. Dr. Xuemei Zhang, melon and watermelon breeder for Hollar Seeds in Colorado, says, “Powdery mildew resistance is my main goal to breed into new melons.” She tries to combine as many races as possible so resistance will not break down in the field. Zhang finds that in hybrids, stronger resistance results when both parents are resistant. “With field resistance,” she adds, “the powdery mildew symptoms won’t set back yield or sugar levels.” Their new releases reflect this concern. Regarding controls, Dr. Kevin Crosby, Texas A&M, says, “There is no silver bullet. As a plant breeder, I consider genetic resistance to be a better approach than fungicide.” He continues work breeding powdery and downy mildew-resistant melons. A just-released Western cantaloupe resists powdery mildew races 1 and 2.

Strategies for Cucurbit Powdery Mildew
Planting resistant varieties reduces the need for fungicides and decreases the chances for fungicide loss of effectiveness. Provide proper nutrition and sanitation. Healthy, vigorous plants can be less susceptible. Scout regularly, checking both upper and lower leaves. Powdery mildew develops early in fruit production. Learn which fungicides are at risk and their mode of action. Start fungicide applications promptly if powdery mildew is detected. The threshold for controls is if one leaf in 45 or 50 shows signs of the disease. The best time to use the at-risk fungicides is early, when the pathogen population is still small. Follow label directions exactly. Alternate fungicides in different FRAC groups. Tank mix at-risk fungicides with protectant fungicides. Maintain an application schedule; seven-day intervals may be necessary. Inspect the effectiveness of the fungicide on the lower leaf surfaces after application. Promptly report poor control to your local extension agent so fungicide resistance can be tracked. Disk early affected crops as soon as possible after harvest to destroy the inoculum. Monitor extension newsletters, university forecasting systems and web postings for updates.

Fungicide management

Fungicide resistance has been problematic for decades. For example, benomyl was registered in 1972; the first case of failure occurred the following year. Since then, other fungicides have failed within a year or two as well. The virulence of powdery mildew pathogens, along with their ability to withstand fungicides, underscores the need for careful management. Several types of resistance exist. When a single major gene modifies, complete loss of control results. In this “qualitative resistance,” higher rates and more frequent fungicide applications are ineffective. When resistance results from modification of several interacting genes, control erodes but greater amounts of the fungicide work initially, but can eventually fail. This is “quantitative resistance.” In addition, fungicides within the same chemical group can all cause resistance. This “cross resistance” can be qualitative or quantitative. Today’s fungicides include chemical controls with differing modes of action, more environmentally sensitive formulations and those designed for preventative controls. McGrath reports, “Fungicides can be highly effective for powdery mildew, especially when the products used are selected based on current information about occurrence of fungicide resistant strains, as well as performance of new products and old ones that are at risk for resistance development.” Two keys are proper timing and correct alternation of fungicide types. Dr. Gerald Brust, IPM vegetable specialist, University of Maryland, observes, “Powdery mildew has been getting worse. Strains have become resistant to many of the fungicides that just a few years before gave us such good control. Growers need to be sure to rotate the fungicides that they use or they will continue to run into problems.”

The 2008 edition of Pennsylvania’s “Commercial Vegetable Production Recommendations” explains how resistance begins. In the disease management section it states, “Pathogens develop resistance to fungicides because of genetic changes in the organism through natural selection or by the intensive use of high-risk fungicides which kill only the susceptible individuals in a population, leaving resistant populations to reproduce and cause more disease.” The recommendations include cautioning against overspraying, using the correct rate, rotating fungicides with those of different modes of action and not using high-risk fungicides as a rescue treatment.

The guide also says, “Applying high or at-risk fungicides only after a disease is present in a field increases the chances for the development of resistant populations of plant pathogenic fungi.”

Dr. McGrath echoes, “It is more difficult, often impossible, to control the pathogen in an established lesion, as opposed to a germinating spore. Thus, the potential is greater for resistance to develop.”

Penn State’s late Professor Alan MacNab said at many grower meetings, “You must start application as soon as disease is noted.”

Managing fungicide resistance

Dr. McGrath reports that most fungicides being developed today have a single-site mode of action because this is associated with lower potential for negative impacts on the environment, including nontarget organisms. These are at a higher risk for resistance development than those with multisite modes of action. Presence of fungicide-resistant strains of pathogens has unfortunately resulted in control failures. Consequently, it is critical in fungicide resistance management to alternate applications of chemicals in different group codes; i.e., ones with different modes of action. The Fungicide Resistance Action Committee (FRAC) designated codes for chemical groups. These codes usually appear on the label front. A list can be found on the FRAC Web site, www.frac.info/frac, in the “Publications” section. Fungicides with the same group code have similar modes of action. Information on a fungicide’s risk is also on the site. Because powdery mildew resistance to many chemicals is widespread, an effective integrated approach includes protectant fungicides. These do not have a high risk for resistance development, since they typically have multisite modes of activity. This delays resistance to the mobile fungicides. Chlorathalonil, copper and sulfur are often recommended, although sulfur can be phytotoxic to certain melon varieties. Mineral oils, botanical oils and a host of other materials can be part of the mix. Many different formulations are currently available for most of these. Some can be used in organic production. Since resistance is constantly evolving, new products are being developed and regional conditions play a role, growers must keep in touch with the local extension agent for the latest guidelines. Regardless of treatment used, label directions must be followed closely.

The author is a writer/researcher specializing in agriculture. She currently resides in central Pennsylvania.