Meg McGrath, Cornell University
Powdery mildew is the most common disease of cucurbit crops occurring every year throughout most of the USA. It is caused by Podosphaera xanthii. The characteristic white, powdery fungal growth is readily recognizable on leaves, stems and vines. This growth is mostly spores that are easily dispersed by wind. Symptoms typically are most severe on the lower surface of leaves. Symptoms can be difficult to see on watermelon, however, as spores are produced less abundantly than on other cucurbits.
Powdery mildew on pumpkin. Photo credit: Meg McGrath, Cornell University
Uncontrolled powdery mildew indirectly affects yield because infected leaves usually wither and die. Premature loss of leaves can result in reduced market quality because fruit become sunburnt, have poor color, or have low sugar content due to premature or incomplete ripening. Fruit with low sugars have poor flavor and poor storability. Handles on pumpkin fruit may be shriveled or rotten in addition to fruit being paler orange. Size and/or number of fruit can be reduced in summer squash and in other crops when powdery mildew is severe. Severe disease can also lead to imperfections on fruit rind such as speckling and oedema (small, raised, wart-like tissue). In addition, powdery mildew infection predisposes plants to other diseases, in particular, gummy stem blight (also known as black rot).
Powdery mildew can be managed with resistant varieties and regular foliar applications of fungicides. It is not possible to escape infection because the pathogen produces many wind-dispersed spores, cucurbit crops are grown widely, and conditions often are favorable for this disease. The powdery mildew fungus tolerates a wide range of temperatures below about 100 F and it does not need a period of free moisture on leaves to infect, in contrast with other foliar fungal pathogens. Rain is actually unfavorable for disease development. Removing affected tissue is not likely to be helpful because powdery mildew typically does not start in foci and the action of removing leaves could dislodge spores, further spreading the pathogen. Stressed plants are more susceptible.
There are now varieties of most cucurbit types with genetic resistance to powdery mildew. Tables of varieties are at Vegetable MD Online (vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm). It can be challenging to find organically-produced or non-pesticide-treated seed. Some seed companies with only treated seed listed in the catalogue are willing to obtain non-treated seed when available for large orders or regular customers. Talk directly with seed suppliers and place orders early to obtain untreated seeds if organic seeds of resistant varieties are not available.
Races of the powdery mildew fungus have been differentiated on cantaloupe. With race-specific resistance, it is important to know what races of the pathogen are likely to be present before selecting a variety. If the variety planted does not have resistance to the predominant race present, it may become as severely infected as a susceptible variety. Race 1 and 2 have been common. A new race (S) has been detected recently in Georgia where resistant varieties have been more severely affected by powdery mildew than in the past.
Resistant varieties of cantaloupe and also cucumber can exhibit high levels of resistance with very few symptoms developing. Disease onset is delayed and/or development slowed in resistant squash and pumpkin; however, these varieties can become severely infected by the end of the season. Varieties with resistance from both parents (homozygous) usually develop less powdery mildew than those with resistance from one parent (heterozygous). In catalogues these are often, but not consistently, described as resistant and tolerant, respectively.
The powdery mildew fungus has the potential to evolve so that it can overcome genetic resistance. Selection pressure for new strains will increase as resistant varieties are grown more extensively; therefore, an integrated program with a few foliar applications of NOP-approved fungicides is recommended to delay selection of new races of the pathogen able to overcome the resistant variety, as well as to improve control in squash and pumpkin. Watermelon was considered naturally immune, but recently crops have been affected, often severely, from Texas to New England, indicating development of a new pathogen strain. Considering how few major genes for resistance are presently incorporated into crops through traditional breeding (almost all squash and pumpkin varieties have the same gene), and the pathogen’s proven ability to evolve new races able to infect resistant melon varieties, it is prudent to use an integrated management program to preserve effectiveness of resistant varieties.
Routine scouting is needed to ensure fungicide applications are started very early in powdery mildew development. Plants are susceptible to powdery mildew when in their reproductive phase and at any age when grown under greenhouse conditions. Sometimes symptoms begin to develop on field-grown plants before they begin producing fruit, especially when severely stressed due to delayed transplanting or competition from numerous tall weeds; removing these stresses can delay powdery mildew development thereby avoiding the need to start applications early. Inspect plants weekly, especially when the first fruits start to enlarge. When first fruit start to enlarge is an especially important time. The scouting protocol entails weekly examination of top and underside of leaf surfaces of 5 old, crown leaves in at least 10 locations throughout a field. Symptoms develop first on older leaves, and often on the underside. It is time to start applications when powdery mildew is found at a very low level. The threshold is 1 leaf with symptoms out of 50 leaves. Once symptoms are easily seen without hunting, potential to effectively manage powdery mildew with rescue treatments is greatly diminished. Due to differences in susceptibility, scout each variety individually. For more information on scouting, see Scouting for Vegetable Diseases on Your Organic Farm.
The pathogen develops best on the lower surface (underside) of leaves, thus a successful management program necessitates managing the pathogen on the lower as well as the upper surface to avoid premature death of leaves. Unfortunately there are no products with systemic activity approved for organic production and it is difficult to directly deliver fungicide to the lower leaf surface, even with new nozzle types and air assist sprayers. Sulfur has been one of the more effective products, which may partly reflect the fact that sulfur can volatilize and thus may be redistributed to the lower surface. Sulfur is also less expensive than other materials. Like copper, sulfur is an element and thus cannot be degraded and removed from the environment as can materials such as oils. Sulfur is a micronutrient for plants. Note that sulfur can be phytotoxic on melons, especially if applied when temperatures are hot.
NOTE: Before applying ANY pest control product, be sure to: 1) read the label to be sure that the product is labeled for the crop you intend to apply it to and the disease you intend to manage, 2) read and understand the safety precautions and application restrictions, and 3) make sure that the brand name product is listed your Organic System Plan and approved by your certifier. For more information on how to determine whether a disease management product can be used on your farm, see Can I Use this Product for Disease Management on my Organic Farm? Make sure to keep label information and records of all input applications.
Several products approved for organic production have been effective for powdery mildew when evaluated in replicated experiments. Efficacy for providing full-season control ranged from poor to at least as good as the popular conventional protectant fungicide chlorothalonil formulated as Bravo (this product is not allowed for use on organic farms). Effective products include:
For more information see the related eOrganic article Efficacy of Microbial Biopesticides that may be used in Organic Farming.
To ensure good yield of high quality fruit of cucurbit crops, manage powdery mildew by:
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.