ON VEGETABLES

Integrated Pest Management for Home Gardeners

Powdery mildew is a common disease on many types of plants. There are many different of powdery mildew fungi (such as species, Sphaerotheca species) and each species only attacks specific plants. A wide variety of vegetable crops are affected by powdery mildews, including artichoke, beans, beets, , , eggplant, lettuce, melons, , peas, peppers, pumpkins, radicchio, radishes, squash, toma- tillo, tomatoes, and turnips (Table 1). Powdery mildews generally do not require moist conditions to establish Figure 1. Powdery mildew on pumpkin vine. and grow, and normally do well under warm conditions; thus they and fruit that has little flavor. mildew fungi are killed and germina- are more prevalent than many other tion is inhibited by water on plant sur- leaf-infecting diseases under Califor- LIFE CYCLE faces for extended periods. Moderate nia’s dry summer conditions. All powdery mildew fungi require temperatures (60° to 80°F) and shady living plant tissue to grow. Year-round conditions generally are the most IDENTIFICATION AND availability of crop or weed hosts is favorable for powdery mildew devel- DAMAGE important for the survival of some opment. Spores and fungal growth are Powdery mildew first appears as white, powdery mildew fungi. Special rest- sensitive to extreme heat (above 90°F) powdery spots that may form on both ing spores are produced, allowing and direct sunlight. surfaces of leaves, on shoots, and overwinter survival of the species that sometimes on flowers and fruit (Fig. causes the disease in cucurbits, lettuce, MANAGEMENT 1). These spots gradually spread over a peas, and certain other crops. The best method of control is preven- large area of the leaves and stems. An tion. Planting resistant vegetable vari- exception is one of the powdery mil- Most powdery mildew fungi grow as eties when available, or avoiding the dews that affects artichokes, onions, thin layers of mycelium (fungal tissue) most susceptible varieties, planting in peppers, and tomatoes: it produces on the surface of the affected plant the full sun, and following good cul- yellow patches on leaves but little part (Fig. 2). Spores, which are the tural practices will adequately control powdery growth. primary means of dispersal, make up powdery mildew in many cases (Table the bulk of the white, powdery growth 1). However, very susceptible veg- Leaves infected with powdery mildew visible on the plant’s surface and are etables such as cucurbits (cucumber, may gradually turn completely yellow, produced in chains that can be seen melons, squash, and pumpkins) may die, and fall off, which may expose with a hand lens; in contrast, spores require fungicide treatment. Several fruit to sunburn. On some plants, of downy mildew grow on branched least-toxic fungicides are available but powdery mildew may cause the stalks that look like tiny trees. must be applied no later than the first leaves to twist, buckle, or otherwise sign of disease. distort. Powdery mildew fungal Powdery mildew spores are carried by growth does not usually grow on wind to new hosts. Although humidity Resistant Varieties vegetable fruits, although pea pods requirements for germination vary, all In some cases, varieties resistant to may get brownish spots. Severely powdery mildew species can germi- powdery mildew may be available. infected plants may have reduced nate and infect in the absence of free If available, plant resistant varieties yields, shortened production times, water. In fact, spores of some powdery of cantaloupe, cole crops, cucumber,

PEST NOTES Publication 7406 University of California Agriculture and Natural Resources November 2008 November 2008 Powdery Mildew on Vegetables

Table 1. Host Plants and Control Measures for Powdery Mildew Species.

Hosts species Controls

, endive, lettuce, melons, Erysiphe cichoracearum resistant varieties of lettuce, cucumber; water sprays; potato, pumpkin, squash fungicides if necessary on squash and pumpkin broccoli, Brussels sprouts, cauliflower, and Erysiphe cruciferarum not usually required other cole crops; radicchio, radishes, turnips tomatoes Erysiphe lycopersici fungicides if necessary peas Erysiphe pisi resistant varieties; sprinkler irrigation , , parsnips Erysiphe heraclei tolerant varieties beets Erysiphe polygoni tolerant varieties artichoke, eggplant, peppers, tomatillo, tomatoes Leveillula taurica rarely required; fungicides if necessary beans, black-eyed peas, cucurbits, okra Sphaerotheca fuliginea resistant varieties for some; fungicides if necessary melons, peas, pumpkins, and squash. Fungicides. Several least-toxic fungi- have some protectant activity. If you plant more susceptible varieties, cides are available, including horticul- you may need to take control mea- tural oils, neem oil, oil, sulfur, Oils. To eradicate mild to moderate sures. and the biological fungicide Serenade. powdery mildew infections, use a hor- With the exception of the oils, these ticultural oil such as Saf-T-Side Spray Cultural Practices materials are primarily preventive. Oil, Sunspray Ultra-Fine Spray Oil, Plant in sunny areas as much as pos- Oils work best as eradicants but also or one of the plant-based oils such as sible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow- release fertilizer. Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers released in spring rains are not usually recommended as a control method in vegetables because their use may contribute to other pest conidia reinfect crop problems.

Fungicide Application ascus containing In some situations, especially in the ascospores production of susceptible cucurbits, spores infect tissue fungicides may be needed. Fungicides function as protectants, eradicants, or both. A protectant fungicide pre- vents new infections from occurring whereas an eradicant can kill an infections on leaves and shoots produce existing infection. Apply protectant conidia (spores) fungicides to highly susceptible plants before the disease appears. Use eradi- cants at the earliest signs of the dis- ease. Once mildew growth is extensive, overwinters in debris control with any fungicide becomes more difficult. The products listed powdery white patches appear here are for home garden use. Com- on leaves mercial growers should consult the UC Pest Management Guidelines, which cleistothecia may be are available online at the following produced on leaves and shoots in late summer address: http://www.ipm.ucdavis .edu/PMG/crops-agriculture.html. Figure 2. Powdery mildew life cycle on squash.

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neem oil or jojoba oil (such as E-rase). some squash and melon varieties. To susceptible plant parts. As plants grow Be careful, however, to never apply an oil avoid injuring any plant, do not apply sul- and produce new tissue, additional spray within 2 weeks of a sulfur spray or fur when air temperature is near or over applications may be necessary at 7- to plants may be injured. Also, oils should 90°F and do not apply it within 2 weeks 10-day intervals as long as conditions never be applied when temperatures are of an oil spray. Other sulfur products, are conducive to disease growth. above 90°F or to drought-stressed plants. such as sulfur dust, are much more Some plants may be more sensitive difficult to use, irritating to skin and If mild to moderate powdery mildew than others, however, and the inter- eyes, and limited in terms of the plants symptoms are present, the horticul- val required between sulfur and oil they can safely be used on. Copper is tural oils and plant-based oils such as sprays may be even longer; always also available to control powdery mil- neem oil and jojoba oil can be used to consult the fungicide label for any dew but is not very effective. reduce or eliminate the infection. special precautions. Biological Fungicides. Biological REFERENCES Sulfur. Sulfur products have been fungicides (such as Serenade) are Flint, M. L. 1998. Pests of the Garden and used to manage powdery mildew for commercially available beneficial Small Farm: A Grower’s Guide to Using Less Pesticide. Oakland: Univ. Calif. centuries but are only effective when microorganisms formulated into a Agric. Nat. Res. Publ. 3332. applied before disease symptoms product that, when sprayed on the appear. The best sulfur products to plant, destroys fungal pathogens. The Gubler, W. D., and D. J. Hirschfelt. use for powdery mildew control in active ingredient in Serenade is a 1992. Powdery Mildew. In Grape Pest gardens are wettable sulfurs that are bacterium, , that helps Management. Oakland: Univ. Calif. specially formulated with surfactants prevent the powdery mildew from Agric. Nat. Res. Publ. 3343. pp 57-63. similar to those in dishwashing deter- infecting the plant. While this product gent (such as Safer Garden Fungicide) functions to kill the powdery mildew organism and is nontoxic to people, McCain, A. H. 1994. Powdery Mildew. However, sulfur can be damaging to pets, and beneficial insects, it has not HortScript #3, Univ. Calif. Coop. Ext. ❖ For more information contact the University proven to be as effective as the oils or Marin County. of California Cooperative Extension in your sulfur in controlling this disease. county. See your telephone directory for addresses and phone numbers. How to Use. Apply protectant fun- AUTHORS: R. M. Davis, , gicides, such as wettable sulfur, to UC Davis; W. D. Gubler, Plant Pathology, UC susceptible plants before or in the Davis; S. T. Koike, UCCE Monterey Co. earliest stages of disease development. TECHNICAL EDITOR: M. L. Flint The protectant fungicides are only COORDINATION & PRODUCTION: P. N. Galin ILLUSTRATIONS: Fig. 1: C. M. Dewees; effective on contact, so applications Fig. 2: adapted from Grape Pest Manage- must provide thorough coverage of all ment, Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3343. Leaves by C. M. Dewees. WARNING ON THE USE OF CHEMICALS Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, Produced by UC Statewide IPM Program, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. University of California, Davis, CA 95616 Pesticides applied in your home and landscape can move and contaminate creeks, rivers, and oceans. Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens This Pest Note is available on the containing fruits or vegetables ready to be picked. World Wide Web (www.ipm.ucdavis.edu) Do not place containers containing pesticide in the trash or pour pesticides down sink or toilet. Either use the pesticide according to the label or take unwanted pesticides to a Household Hazardous Waste Collection site. Contact your county agricultural commissioner for additional information on safe container disposal and for the location of the Household Hazardous Waste Collection site nearest you. Dispose of empty containers by following label directions. Never reuse or burn the containers or dispose of them in such a manner that they may contaminate water supplies or natural waterways.

The University of California prohibits discrimination or harassment of any person on the basis This publication has been anonymously peer of race, color, national origin, religion, sex, gender identity, pregnancy (including childbirth, and reviewed for technical accuracy by University of medical conditions related to pregnancy or childbirth), physical or mental disability, medical condition California scientists and other qualified profession- (cancer-related or genetic characteristics), ancestry, marital status, age, sexual orientation, citizen- als. This review process was managed by the ANR ship, or service in the uniformed services (as defined by the Uniformed Services Employment and Associate Editor for Urban Pest Management. Reemployment Rights Act of 1994: service in the uniformed services includes membership, applica- To simplify information, trade names of products tion for membership, performance of service, application for service, or obligation for service in the have been used. No endorsement of named products uniformed services) in any of its programs or activities. University policy also prohibits reprisal or is intended, nor is criticism implied of similar products retaliation against any person in any of its programs or activities for making a complaint of discrimi- that are not mentioned. nation or sexual harassment or for using or participating in the investigation or resolution process This material is partially based upon work supported of any such complaint. University policy is intended to be consistent with the provisions of appli- by the Extension Service, U.S. Department of cable State and Federal laws.Inquiries regarding the University’s nondiscrimination policies may be Agriculture, under special project Section 3(d), directed to the Affirmative Action/Equal Opportunity Director, University of California, Agriculture and Integrated Pest Management. Natural Resources, 1111 Franklin Street, 6th Floor, Oakland, CA 94607, (510) 987-0096.

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