Heads up – consider the potential for fire blight in your orchard and act NOW.

We are experiencing a period of extreme fire blight risk in many apple-growing areas of Michigan, but especially in southwest Michigan for the period May 12-15, 2011. Maryblyt epiphytic infection potential (EIP) values are higher than any we’ve observed in a number of years. Remember that the EIP addresses risk in orchards with open bloom. We’ve had enough warm days now in most areas that the fire blight pathogen should be activated and beginning to ooze from cankers. The flower is the site where the pathogen grows to extremely large numbers under conducive conditions. The environmental conditions this year are optimal for pathogen growth and fire blight infection.

For orchards with blocks of susceptible varieties in regions affected by streptomycin resistance, Kasumin is the antibiotic of choice for blossom blight management. See my previous article for details on Kasumin use.

It is inevitable that orchards will sustain some blossom blight infection this year. With predicted inoculum loads of the pathogen at epidemic levels, we can’t expect optimal control. What else can be done? 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 southwest Michigan, we will likely be there this weekend. 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 10-14 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 3 to 4 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.

I am suggesting two things for 2011: (i) that growers use Apogee and not miss the king bloom petal fall timing; and (ii) that growers in extreme risk areas 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 12-16 oz / A, 8 oz / A, and 8 oz / A this year. (See page 216 of MSUE’s 2011 Michigan Fruit Management Guide for more information on Apogee rates and usage).

Extreme risk areas for 2011 include areas with trees past 50 percent bloom by May 14, 2011. This would include southwest Michigan and many orchards on the Ridge. Temperatures are predicted to cool into next week. Trees that are not at >50% bloom until into next week will not be subject to as significant a fire blight risk, and normal Apogee application strategies can be used.

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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