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Management of Powdery Mildew and Phytophthora Fruit Rot, Two Important Cucurbit Diseases 1997

Project Leaders: Margaret Tuttle McGrath, Nina Shishkoff, Joseph B. Sieczka


1. Evaluate fungicides applied to pumpkin after disease detection for control of powdery mildew.

2. Determine whether race 2 or race 3 of the powdery mildew fungus is present on Long Island.

3. Evaluate four powdery mildew-resistant (PMR) experimental pumpkin varieties.

4. Compare susceptibility to Phytophthora fruit rot of hard-rinded and conventional pumpkins.

5. Evaluate oat straw mulch and ryegrass living mulch for control of Phytophthora fruit rot.

6. Evaluate cocoa shell mulch, rye straw mulch, and rye stubble for control of Phytophthora fruit rot.


Objective 1: Equivalent control to that of the "standard" fungicide program (Bravo applied on a 7-day schedule plus three systemic fungicides) was obtained at a lower cost by substituting copper fungicides for Bravo and by applying Bravo on a 14-day schedule. Copper and sulfur applied alone on a 7-day schedule were quite effective on upper leaf surfaces and moderately effective on lower leaf surfaces.

Fungicide resistance to the two currently registered systemic fungicides continues to be a problem. Efficacy data were obtained to support a Section 18 registration request for two systemic fungicides, Nova and Quadris. The latter, applied alone on a 14-day schedule, was as effective as the "standard" fungicide program on a 14-day schedule. Advantages of Quadris are that it is considered by the EPA to be a reduced-risk fungicide and that it has broad-spectrum activity against several diseases.

Several biocompatible fungicides were also tested. Three of them (potassium bicarbonate, monopotassium phosphate foliar fertilizer, and Trichoderma) partially suppressed powdery mildew but were not as effective as conventional fungicides.

Objective 2: Race 2 but not race 3 of the powdery mildew fungus occurred on Long Island. Race 1 may have also been present.

Objective 3: The four PMR pumpkins evaluated produced fruit of very high quality but did not exhibit the level of disease control expected.

Objective 4: The two experimental pumpkins with hard rinds like gourds were much less susceptible than the other two experimentals tested.

Objective 5: Pumpkin plants in the oat straw plots grew more slowly and the fruit turned orange later than in the other plots. Phytophthora developed in all of the plots, but it developed more slowly than expected under frequent irrigation. Almost all fruit rot due to Phytophthora started where fruit touched soil. There were no significant difference among the treatments.

Objective 6: Disease did not differ significantly among treatments due to great variability in disease incidence from one side of the field to the other. However, it was clear that black plastic mulch developed much higher levels of disease than other plots in parts of the field where the disease occurred, presumably because it was conducive to splashing during irrigation and rain. The cocoa shell mulch had a better overall combination of high yield, few weeds, and low disease than did the other mulches.