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Powdery Mildew Control Using Low Risk Pesticides and Effective Application Procedures in Papaya

Grant Recipient:

Department of Plant and Environmental Protection Sciences
University of Hawaii
Honolulu, Hawaii 96822

Project Period:

14 months: August 1998 to October 1999

Principal Investigator:

Janice Y. Uchida and Michael Kawate

Crop production in Hawaii is severely impacted by diseases caused by powdery mildews. Papaya fruit production is worth over $18 million to the state and the fruit is widely grown in many tropical and subtropical areas. Recently, papaya growers on Hawaii, Molokai and Kauai reported severe damage to papaya trees by powdery mildew. In many cases, mites are also a problem. Growers believe that the standard control for mildew using sulfur is not working. Claims have been made that the mildew is "different" and other growers are concerned that mildew from other crops is spreading to papaya. The Papaya Administrative Council and many growers are interested in demonstrations of the efficacy of sodium or potassium bicarbonate and azoxystrobin (Quadris). Although sulfur has been shown to control mildew, efficacy is dependent on good coverage and retention of the sulfur on leaves. The major goal of this project is to demonstrate whether sulfur will provide effective control of powdery mildew if properly applied and whether bicarbonate and azoxystrobin are also effective.

Spray applications were made on trees in fields. In general, spray were applied with a gas-powered backpack mist blower. Treatments were made weekly or on alternate weeks. Disease evaluations were made by cisually estimating the number of colonies or percent leaf area infected with mildew and a rating scale was developed.

Project goals and objectives
1). Demonstrate if sulfur is effective in controlling powdery mildew on papaya.
2). Demonstrate if the addition of spreaders/stickers increase disease control. If possible, test different rates to reduce potential phytotoxicity.
3). Demonstrate effectiveness of potassium bicarbonate. Determine if this compound is phytotoxic.
4). Demonstrate effectiveness of Quadris (azoxystrobin), a fungicide, showing potential for disease control in early greenhouse tests.

Overall, the level of powdery mildew at ITC, Kauai, was relatively low during the test period. Plants appeared healthy and were not losing leaves to mildew. Sulfur appeared to be effective at keeping powdery mildew and mites to manageable levels.

The only effective treatment for powdery mildew control appeared to be Rally. With respect to colony development on treated leaves, the reduced-risk fungicides did not appear effective. However, colonies on the leaves treated with Quadris and fortress appeared less powdery (less conidia formed, although this was not quantified.

Potassium bicarbonate did not appear to be highly effective but was not phytotoxic to papaya leaves or fruits.

Project Links

American Farmland Trust