MSU Extension

Over the last week or two, blueberry leaf rust has been noticed in blueberry fields in various locations in Michigan. Blueberry leaf rust has been on the increase in Michigan in recent years, due to favorable weather conditions for the disease over the past years.Rainy periods in mid- to late summer as well as longer dew periods are conducive to disease development. It also occasionally shows up on blueberry plants grown in the greenhouse.

Leaf rust is caused by the fungus Thekopsoraminima. Small yellow spots appear on leaves between July and September and then turn brown with a darker border (view images). On older leaves, lesions may be surrounded by red or purplish discoloration or, when leaves are about to senesce, rust spots may actually be surrounded by green areas (“green island effect”) as the fungus needs the leaf to stay alive to be able to reproduce itself.

On the lower leaf surface, yellow to orange powdery pustules (uredia) are visible with the naked eye or hand lens. These contain infective spores and, when touched, the spores come off.

Leaf rust can rapidly increase towards the end of the season under warm, wet conditions. It generally has little impact on current-season yield, but may cause premature defoliation. Severe defoliation has the potential to reduce fruitfulness of new buds and winter-hardiness of the canes. No studies have been done on yield loss in blueberries due to leaf rust.

Some growers believe that low yields observed in 2011 were caused by leaf rust. This is not necessarily the case; low yields may have more to do with environmental conditions affecting bud set. Heavy crop load one year may also affect fruitfulness the following year. Unless you observed rust pustules and associated premature defoliation, you cannot assume that rust was the cause of low yield potential. In addition, other factors can lead to premature defoliation, such as drought stress, chemical injury, Phytophthora root rot and Armillaria root rot. If you see reddish or yellowish leaves dropping without orange rust pustules, it is most likely not leaf rust.

Biology

The disease has probably been present in Michiganfor a long time on wild blueberries. The rust fungus needs a live host to perpetuate itself. The alternate host of the fungus is the hemlock (Tsugaspp.), which explains why the rust tends to be more severe in the vicinity of hemlock trees. The hemlock plays an essential part in the life cycle of the fungus.

In spring, spores produced in overwintered blueberry leaves on the ground below blueberry bushes become airborne and infect hemlock needles. This infection may be difficult to see but infected needles turn yellow and have cream-colored tube-like projections hanging from them on the lower surface. Spores from these are airborne and infect blueberry leaves anytime from July to August. Once blueberry leaves are infected, spores produced in the rust pustules are spread by wind and re-infect blueberry leaves in the presence of water from rain, dew, or overhead irrigation.

There can be repeated cycles of infection as long asconditions are conducive. In the fall, the fungus drops to the ground with theleaves and overwinters in the field until the next growing season. In regionswhere green leaves are present year round (e.g., in the southern United States, Mexico or greenhouses), hemlock trees are not needed to close the life cycle.

Management

Removing all hemlock trees within a half mile from a blueberry field would break the rust life cycle but is neither desirable nor practical. Besides, hemlocks are a beautiful part of the Michigan landscape.

Raking or vacuuming and burning blueberry leaves after leaf drop in the fall can reduce inoculum carry-over. Minimize leaf wetness by adjusting timing of overhead irrigation and apply effective fungicides in mid- to late summer. Even though fungicides have not been tested for blueberry rust control in Michigan, the sterol inhibitors are usually highly effective against rusts. At this time, most of the blueberry harvest has been completed, which is important as the most effective fungicides have long pre-harvest intervals. Both Indar (fenbuconazole) and Orbit (propiconazole)are labeled for rust control in blueberries and have a 30-day PHI. Bravo (chlorothalonil) is also labeled for rust control and has a 42- day PHI.However, after harvest, all of these fungicides can be sprayed provided that the maximum number of sprays per season is not exceeded.

A tankmix of a sterol inhibitor fungicide (e.g., Indar or Orbit) with a half rate of Bravo is another option for both protectant and curative activity. Sonata (Bacillus pumilis), an organic biofungicide with a 0-day PHI is also labeled for blueberry rust control – adding a spreader-sticker may improve efficacy. Cabrio (pyraclostrobin) and Pristine(pyraclostrobin + boscalid) are labeled for rust “suppression” and are not recommended for rust control except during the harvest season when other products cannot be used. Dormant lime sulfur applied to the overwintering leaves on the ground in the fall or spring may be helpful but efficacy has not been confirmed.