Extreme fire blight risk in northwest Michigan

With epiphytic infection potential levels well over 200 in the north and severe weather forecast for later today (May 31), the risk of fire blight infection is great.

We are experiencing a period of extreme fire blight risk in northwest Michigan for apple growers for the period of May 31-June 7. Maryblyt epiphytic infection potential (EIP) values are well above 200 today (May 31) and predicted to remain high for the next week. The EIP addresses risk in orchards with open bloom or where potential trauma may occur (severe weather is predicted the afternoon of May 31). At this time, streptomycin remains the bactericide of choice for controlling fire blight in the northwest, however, if you are located in Grand Traverse County and have streptomycin resistance, Kasumin may be applied. See George Sundin’s article, Section 18 Special Exemption Lable for Kasumin for Fire Blight Control In 2011, for more information on Kasumin use.

The most important next control measure is to use Apogee (prohexadione calcium) for shoot blight management. Apogee is a growth inhibitor that provides excellent control of shoot blight. The first timing for an Apogee spray is at king bloom petal fall. In northwest Michigan, we will likely be there this week in most locations and varieties. Apogee is shoot specific, i.e. the effect is only observed if the shoot is covered; thus excellent coverage is essential. The “Apogee effect” on fire blight begins approximately 10 to 14 days after application, and research from my lab suggests that it is associated with cell wall thickening in apple shoots.

Most growers prefer to space Apogee applications out using three to four applications (once every two weeks.) This strategy is effective for shoot growth control and fire blight management under low and moderate disease pressure. However, under very high disease pressure such as what we are experiencing this year, use of a higher rate application provides better shoot blight control than lower rates.

There are two management strategies that can help growers combat fire blight in the coming days: use Apogee and do not miss the king bloom petal fall timing; and growers in extreme risk areas should increase the rate used for their first application by at least 50 percent and consider doubling the rate for the first application. Return to the usual rate of Apogee used in the second and third applications. For example, growers that use rates of 8 oz/A, 8 oz/A, and 8 oz/A for their three Apogee applications in typical years should go 12 to 16 oz /A, 8 oz/A, and 8 oz/A this year. (See page 216 of MSU Extension’s 2011 Michigan Fruit Management Guide for more information on Apogee rates and usage).

Apogee must be used with an organosilicone surfactant and an equal weight of spray grade ammonium sulfate should be applied. Do not use Apogee on “Empire” or “Winesap” because of the potential for fruit cracking.

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

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