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GUIDELINES FOR MANAGING CUCURBIT POWDERY MILDEW ORGANICALLY Margaret Tuttle McGrath Department of Plant Pathology, Cornell University Long Island Horticultural Research and Extension Center; 3059 Sound Avenue Riverhead, NY 11901; [email protected]; vegetablemdonline.ppath.cornell.edu Powdery mildew is the most common disease of cucurbit crops occurring every year throughout the US. The characteristic white, powdery fungal growth is readily recognizable on leaves and vines. This growth is mostly spores that are easily dispersed by wind. Symptoms can be difficult to see on watermelon, however, as spores are produced less abundantly than on other cucurbits. 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 ripening prematurely or incompletely. 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. In addition, powdery mildew infection predisposes plants to other diseases, in particular, gummy stem blight (aka black rot). Powdery mildew can be managed with resistant varieties and regular foliar applications of disease control products and 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. There are now varieties of most cucurbits types with genetic resistance to powdery mildew (Table 1). Unfortunately it can be challenging to find organically-produced or non-pesticidetreated seed. Some seed companies are willing to obtain non-treated seed when available for large orders or regular customers. 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 a race is present that the variety does not have resistance for it may become as severely infected as a susceptible variety. Both race 1 and 2 have been present recently on Long Island, NY. 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 described as resistant and tolerant, respectively. The powdery mildew fungus has the potential to evolve such 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 is recommended to thwart 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. The fungus causing powdery mildew is sensitive to many types of chemicals that are approved for organic production. These include oil (mineral and botanical types), sulfur, copper, potassium bicarbonate, and biofungicides (Table 2). Efficacy and cost vary widely (Table 3). Note that copper is the only product labeled for several other diseases. For effective control, applications need to be started early in disease development and repeated every 7 to 10 days (14 days with resistant varieties). Established powdery mildew cannot be controlled, not even with systemic conventional fungicides. Routine scouting is needed to ensure 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. These are critical times to examine plants. Start in the field when first fruit start to enlarge. The scouting protocol entails weekly examining both leaf surfaces of 5 old, crown leaves in at least 10 locations through out a field. Symptoms develop first on older leaves, often on the underside. It is time to start applications when powdery mildew is found at a very low level on at least 1 of the 50 leaves. Once symptoms are easily seen without hunting, potential to effectively manage powdery mildew with rescue treatments is greatly diminished. Symptoms have also been observed on young field-grown plants that were severely stressed due to delayed transplanting or surrounded by tall weeds; removing these stresses can halt powdery mildew development thereby avoiding the need to start applications early The pathogen develops best on the lower surface (underside) of leaves, thus a successful management program necessitates controlling 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. Foliar applications of sulfur are more effective than most other organic products for powdery mildew on lower surfaces apparently because when deposited on the upper leaf surface, sulfur can volatilize and 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. In summary, to ensure good yield of high quality fruit of cucurbit crops, manage powdery mildew by selecting varieties with resistance, examining foliage weekly to detect when this disease begins to develop, and applying control products to foliage beginning very early in disease development. * note that in the US products for which the manufacturer claims disease control must be registered as fungicides with EPA unless the ingredients are ‘generally recognized as safe’ (GRAS) by EPA. Table 1. Powdery mildew resistant cucurbit varieties. Cucumber, slicers: Autograph, Burpless 26, Calypso, Carolina, Cobra , Competitor , Dasher II, Daytona, Diamonte, Diva, Eureka (F1), Fanfare, General Lee, Genuine (F1), Greensleeves, Impact , Indio, Indy, Intimidator, Jackson Classic, Lafayette Classic, Lynx, Marketmore 76, Marketmore 86, Olympian (F1), Panther, Prancer, Slice More (F1), Socrates (F1), Speedway, SR2389CW, Stonewall, Suyo Long, Sweet Slice, Tasty Bright, Tasty Green, Tasty Jade, Thunder, Thunderbird, Turbo, Tyria (F1), Ultra Pak Cucumber, pickles: Alibi, Amour , Calypso, Classy, Cross Country, Eclipse, Eureka, Fancipak M , Feisty, FM5020 , Jackson Classic, Jackson Supreme, Lafayette, H-19 Little Leaf, Moxie, Napoleon , Patton, Pioneer, Salty, Sassy, SMR58, Spear It, Spunky, Timor, Vlasstar, Wellington, Wisconsin, Zapata Muskmelon: Ambrosia, Aphrodite, Asian, Athena, Burpee Hybrid, Bush Star, Classic, Cresent Moon, Earligold , Eclipse, Harper Hybrid, Minerva, Odyssey, Primo, Pulsar, Saticoy, Sugar Bowl, Sweet’n Early Melon speciality: Arava, Bolero, Cream de Menthe, Crete, Dorado, Edonis, Gallicum, Golden Beauty, Honey Brew, Marygold, Minnesota Midget, Rayyan, Savor, Sweetie #6 (Butterscotch) Pumpkin: Aladdin, Bumpkin, Gold Dust, Gladiator, Gold Boullion, Magician, Pure Gold, Harvest Princess, Super Herc, Harvest Time, Magic Lantern, Hobbit, Merlin, Iron Man, Golden Condor, One Too Many, Scarecrow, Touch of Autumn Summer squash, yellow: General Patton, Patriot II, Sunglo, and Sunray Summer squash, zucchini: Payroll, Sebring Winter squash, acorn: Autumn Delight, Royal Ace, Table Star, Taybelle PM Winter squash, butternut: Bugle, RBX 3106 Winter squash, specialty: Bush Delicata, Celebration, Cornell's Bush Delicata, Harlequin, Metro PMR, Sweet Lightning, TipTop PMR Table 2. Products evaluated for managing powdery mildew organically, active ingredient, whether exempt from tolerance or EPA registration as a fungicide, labeled diseases and insect pests, and company. Note that a few products are not yet OMRI approved or listed. 1. Bugitol. 0.42% Capsaicin and related Capsaicinoids, 3.7% Allyl Isothiocyanate. Not OMRI approved yet. Kills and repels numerous insects. Champon Millennium Chemicals 2. Eco E-RASE. 97.50% Jojoba Oil. OMRI listed. Exempt from tolerance. Controls powdery mildew and white fly. IJO Products 3. GC-3 Organic fungicide. 30% Cottonseed Oil, 30% Corn oil, 23% Garlic extract. Exempt from EPA registration. OMRI listed. Controls powdery mildew. JH Biotech, Inc. 4. Organocide. Active ingredients=5% sesame oil; inerts = 92% fish oil + 3% emulsifiers. Exempt from EPA registration. OMRI listed. Organic Laboratories, Inc. 5. Sporan. 17.6% Rosemary Oil; Other Ingredient = 82.4% Wintergreen Oil. Exempt from EPA registration. OMRI listed. Labeled for several diseases. EcoSmart Technologies. 6. Trilogy. 70% clarified hydrophopic extract of neem oil. OMRI listed. Certis USA L.L.C. 7. JMS Stylet-oil, organic formulation. 97.1% paraffinic oil. OMRI listed. JMS Flower Farms, Inc. 8. Serenade. Bacillus subtilis. Labeled for several diseases. OMRI listed. AgraQuest. 9. Sonata. Bacillus pumilus. Labeled for several diseases. OMRI listed. AgraQuest. 10. OxiDate. 27% hydrogen dioxide. Labeled for several diseases. Will be OMRI listed soon. StorOx, another formulation, is OMRI listed. BioSafe Systems. 11. Kaligreen. 82% Potassium bicarbonate. OMRI listed. Controls powdery mildew. AgBio, Inc 12. MilStop. 85% Potassium bicarbonate. OMRI listed. Controls powdery mildew and other diseases. BioWorks Inc. 13. Champion. Copper hydroxide. OMRI listed. NuFarm Americas Inc. 14. Microthiol Disperss. 80% Sulfur. Will be OMRI listed soon; other sulfur products available. Cerexagri. 15. Nutrol. Monopotassium phosphate (0-50-32). Not OMRI listed. LidoChem, Inc. Table 3. Efficacy of products for managing powdery mildew organically, rate, cost, and percent control achieved in efficacy experiments conducted at Cornell’s research facility on Long Island. The conventional fungicides Bravo, Quadris, Quintec and Procure were included for comparison. Treatments and rate * OxiDate (IPM) 40 fl oz/100 gal OxiDate (curative) 128 fl oz Sporan 1.5 qt/A Trilogy 1% Bugitol 96 fl oz/100 gal Eco E-RASE 1% Organocide 2 oz/gal GC-3 Organic fungicide 1% JMS Stylet-oil 5 qt/100 gal Microthiol Disperss (S) 4 lb/A Bravo Ultrex 2.7 lb/A Quintec + S; Procure + S Application Cost ($/A) 6.41 20.50 29.25 11.75 28.56 30.00 46.88 23.75 7.50 2.64 13.77 Powdery Mildew Control (%) Upper leaf surface 8/23 9/16 95 bc 13 a ** 33 ab 98 c 41 ab 99 c 63 cd 96 bc 62 cd 98 c 76 cd 97 bc 85 cde 96 bc 83 cde 97 bc 77 cd 100 c 92 def 97 bc 76 cd 100 c 99 f Lower leaf surface 8/23 9/16 69 bc 7 ab 21 abc 74 bc 18 abc 83 bcd 21 abc 80 bcd 14 abc 87 cd 28 abc 69 bc 36 bc 80 bcd 42 c 81 bcd 30 abc 95 cd 21 abc 71 bc 18 abc 97 cd 82 d * 8/6-7 (8/11 for Bugitol), 8/12, 8/19, 8/25, 9/1, 9/6, and 9/12-13 (7X). OxiDate curative started with 3 consecutive applications on 8/25, 8/26, and 8/27 when powdery mildew reached a level of being easily seen but not severe (average severity on older leaves was 1% on upper surfaces and 2% on lower surfaces). ** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same letter in a column are not significantly different. Treatments and rate * Champion WP 2 lb/A Sonata AS 2 qt/A Sonata + Champion Compost tea Quintec + S; Procure + Bravo Applic -ation Cost ($/A) $5.70 Powdery Mildew Control (%) Upper leaf surface 8/23 9/16 95 bc 13 a ** 33 ab 33 ab 100 c 99 f Lower leaf surface 8/23 9/16 69 bc 7 ab 21 abc 21 abc 97 cd 82 d * 7/27, 8/4, 8/11, 8/18, 8/24, 8/30, and 9/6 (7X). Compost tea was applied undiluted with NuFilm-P (6 oz/A) at 85 gal/A and 40 psi 9 times from 16 Jul to 8 Sep. Other treatments applied at 100 psi. ** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same letter in a column are not significantly different. Powdery Mildew Control (%) Upper leaf surface Lower leaf surface Fungicide and rate/A 1997 1998 1999 2000 2001 1997 1998 1999 2000 Serenade 6 lb 36 b Prudent Plus 1.4 qt 40 b Milsana 1% 45 b 85 b 59 c 21 ab 26 b Armicarb 100 4 lb 23 a 62 c 23 b Kaligreen 2.2-4 lb 65 ab 62 c 45 bc Nutrol 20 lb 60 ab 66 cd 68 c 29 b 24 b Kocide 2000 2-2.25 lb 98 f-h 72 de 64 b 84 d 60 c-e 20 b 26 ab JMS Stylet-oil 1.5% 93 e Microthiol Disperss 4 lb 99 g-j 96 i 69 b 93 e 72 d-f 63 efg 31 ab Bravo Ultrex 2.7 lb 97 cd 98 e 14 ab Quadris; Nova + Bravo 93 i 100 d 100 e 95 e 58 ef 84 de 98 fg 2001 8 abc 17 bcd 10 a-d 16 bcd 5 ab 21 d 19 cd 55 f 53 f 37 e 72 g ** ‘a’ indicates no control; severity not different from non-treated. Treatments with the same letter in a column are not significantly different.