Early season bacterial canker management in cherry

Dormant sprays must be adequately timed to reduce P. syringae inoculum without causing phytotoxicity.

Bacterial canker is caused by Pseudomonas syringae and can be very damaging to sweet cherries in Michigan. The pathogen is favored by the rainfall and cool temperatures common during Michigan springs. P. syringae bacteria overwinter within the bark tissue in cankers, in asymptomatic buds and internally in the vascular system of the tree. Bacteria multiply in these areas and are disseminated by wind and rain, landing on blossoms and young leaves where they remain until their populations reach a threshold necessary to cause infection. The factors that commonly predispose trees to canker include cold springs, wet weather late in the spring, severe storms that injure emerging blossoms and leaf tissue, and freezing temperatures.

Symptoms of bacterial canker include gummosis from cankers on trunks and branches as well as wilting leaves on the terminal portion of cankered limbs. Leaf and fruit infections usually occur sporadically. Leaf infections appear as dark brown spots and may be circular or angular and surrounded by yellow halos. The spots may blend together over time, causing large areas of dead tissue on the leaves. Lesions on green fruit are brown and look water-soaked around the edges. Affected tissues will collapse, leaving deep black depressions in the flesh as the margins turn from yellow to red.

Early copper sprays are the most common method of control for bacterial canker on cherry. Keep in mind that sweet cherry tissues are extremely sensitive to copper and the sprays must be accurately timed to reduceP. syringae inoculum without causing phytotoxicity. If the trees are still in the dormant stage, two copper applications may be applied at one to two week intervals at a rate of 1.2 lb to 2 lb of metallic copper with either one pint of spray oil per 100 gallons of water or 6 lb to 9 lb of hydrated lime per acre.

According to Rosenberger, copper products sprayed during the dormant stage should have good retention properties to enhance disease control as longer residuals for copper should translate into an extended period of bacterial disease suppression after the spray is applied. If the trees have broken dormancy and are in the pre-bloom stage (bud swell through white bud), copper rates should be reduced to 25 to 35 percent of the dormant rate. Up to two copper applications with a one week interval should be used at this time. In tart cherries, copper compounds can be used at the 1.2 lb to 2 lb actual copper rate at bud burst with weekly repeated applications until late May. Some of these later sprays may result in some leaf yellowing, bronzing and potentially defoliation. Adding hydrating lime at 6 lb to 9 lb/acre will reduce the phytotoxic effects of copper. Do not apply copper at temperatures above 75°F.