Powdery mildew may not be the biggest challenge California melon producers face, but it is one of the most persistent, affecting crops grown from the Sacramento to the Imperial valleys.
Fortunately there are effective fungicides to control the disease and new ones are reaching the market all the time.
How they are used is the most important element of a mildew control program for melons or any crop susceptible to powdery mildew.
Tom Turini, University of California Cooperative Extension plant pathology farm advisor for Imperial County has tested more than a dozen powdery mildew control materials and most are effective. And, growers need every one if they are to practice resistance management, the key element of powdery mildew control.
Resistance has been documented to triazole sterol inhibitor fungicides and to benomyl, which have difference modes of action.
“To reduce the selection pressure for resistance, fungicides with different modes of action should be applied together or alternated,” said Turini.
Sulfur is the only powdery mildew material where resistance has never been detected. However, some melon varieties are susceptible to sulfur burning, said Turini. Sulfur does not offer long residual control like the new fungicides and to be effective it requires thorough coverage.
“We are losing Benlate and that is going to affect resistance management. From what I have been told, when existing stocks are depleted, there will be no more Benlate available,” said the farm advisor. That will mean one more resistance management tool lost.
Mode of action
However, Turini said a fungicide from Dow AgroSciences, Quintec, is expected to help in resistance management. It has a novel mode of action and has shown to be very effective in his trials. It has been used in the Mideast and Europe and no resistance has been detected so far, he added.
Powdery mildew appears as a white, talcum-like growth on the petioles, stems and upper and lower leaf surfaces. Symptoms usually appear first on older, shaded leaves lower in the canopy.
Severely infected leaves wither and die, leaving fruit exposed to sunburn or ripen prematurely or incompletely.
Powdery mildew needs high humidity and that is something not associated with desert melon production. However, Turini said infection can occur when relative humidity is as low as 46 percent, an easily achievable level in irrigated desert melons.
Optimum spore formation temperatures are from 68 to 80 degrees, again normal temperatures for the 10,000 to 15,000 spring melons grown in the Imperial Valley each season. Spores can form and cause infection when temperatures are from 50 to 90 degrees. And Fungi can survive from one season to the next on cucurbits in frost-free areas and on other hosts. Winds can carry spores long distances.
In two years (2000 and 2001) of trials on drip irrigated casaba melons on 80-inch beds, Turini found Procure 50WS, Sovran 50W and Flint 50 WDG provided excellent powdery mildew control. Rally 40W was consistently among the best performing materials. Quintec, Topsin M 70W alone or in combination with Trilogy provided excellent to intermediate levels of control. Cabrio EG and Microthiol Special 80W generally provided intermediate levels of control. Serenade did not consistently improve control as compared to untreated control.
In 2002, Quadris 2.08F performed poorly, said Turini, when applied at seven and 10-day intervals. However, in 2001, it exhibited intermediate levels of control when applied with the surfactant Latron B-1956.
Bravo Ultrex with Quadris 2.08 and Latron B-1956 did not improve control.
Folicure 3.6F provided moderate to poor control in the 2000 experiment.
In the 2001 trial, dusting sulfur provided excellent control. Actigard 50WG provided poor control and two copper compounds, Champ 2F and Solucop, did not reduce disease compared to the untreated control.
In 2001 fungicide programs, where materials with different modes of action were alternated, showed promise, according to Turini. Procure 50WS alternated with Sovran 50W provided excellent control. Topsin M 70W with Trilogy alternated with Flint 50WDG provided acceptable control levels.