Further thoughts on managing brown rot in sweet cherry for 2011

If bacterial canker infection lesions are present in cherries, an intensively active spray program is required from now until harvest.

This article is a follow-up to “Time to scout and control American brown rot in cherries” posted June 22.

Fruit showing bacterial canker infection lesions are highly susceptible to American brown rot infection. Fruit that has been infected by bacterial canker is the initial site where we have first observed American brown rot infections this season. Once the diseased fruit is colonized by the brown rot fungus, they quickly become American brown rot “disease spreaders.” Once the American brown rot fungus begins to sporulate, these fruits are capable of infecting healthy, ripening cherries.

At this time, growers cannot control infections of bacterial canker-infected fruit. The Pseudomonas bacteria have already infected and weakened the fruit, and as a result, these fruits have increased susceptibility to American brown rot. This dead tissue in the canker lesion is highly susceptible to colonization by the American brown rot fungus. There is simply no method available to protect bacterial canker infected fruit from American brown rot infection besides removal from the tree, which is impractical.

All of the other fruit on trees need protection from the relatively small number of cankered fruit infected with American brown rot. An intensively active spray program is required from now until harvest. This program should focus on using Indar with Pristine as an alternative mode of action.

Question: When does 8 = 9 = 10 = 12? These numbers are actually equal when we are talking about rates of Indar 2F for American brown rot control in 2011. Our surveys have shown that over 99 percent of the American brown rot isolates we have recovered in the past three years in Michigan are sensitive to sterol inhibitor (SI) fungicides. These SI-sensitive isolates are adequately controlled with Indar at the 6 fl. oz./A rate. Increasing the rate [24(c) label for Indar 2F only] is a useful hedge, though against the possibility of the presence of SI-resistant strains.

However, for all intents and purposes, use of Indar 2F at 8 fl. oz./A should be just as effective as 12 fl. oz./A because of the presence of little to none SI-resistant American brown rot in Michigan sweet cherry orchards. Thus, the amount of control from a spray of 8 fl. oz./A would be equivalent to that from a spray of 9, 10, or 12 fl. oz./A.

Because the seasonal limit of Indar is 48 fl. oz./A, growers should use rates of 8 fl. oz./A to be able to increase the number of applications available for the duration of the season. The availability of applications of Indar is important as we approach harvest and fruit increases in sugar content and becomes more susceptible to American brown rot infection and because we do not know how many periods of brown rot-conducive weather (temperatures in the 70s with rain) we will experience during this one to three week time. We suggest that growers keep a spray available if needed. If that spray is not needed in the end, growers will minimize input costs.

Growers should maximize the effect of the 8 fl. oz./A application by covering all row middles or keeping a very tight window if using an alternate row strategy (i.e., two to three days maximum). Fruit coverage = fruit protection = American brown rot control.

Additionally, there may be fruit in orchards that is currently infected, but not showing symptoms right now. These symptoms may show up after your next spray, but this does not mean the fungicide application was not effective as the infection was there prior to the spray. The SI fungicides do not have the capability of eradicating American brown rot infections from fruit. This just means that from now to harvest we must maintain a strategy of intensive, active fruit protection to ultimately control American brown rot levels in orchards.

Dr. Sundin’s work is funded in part by MSU’s AgBioResearch.

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