Winter freeze injury has taken its toll on blueberry flower buds with a fairly high proportion of empty buds. With bloom approaching and frequent rains in the forecast, growers may want to protect remaining flower clusters from possible disease attack.

Wet conditions and moderate temperatures may promote blossom and twig blights, particularly Phomopsis twig blight, caused by the fungus Phomopsis vaccinii. The fungus Botrytis cinerea (gray mold) is a minor problem in Michigan blueberries in most years, but may become more prevalent under cooler, wet conditions when blossoms remain covered with moisture for extended periods. Botrytis is an excellent colonizer of dying plant tissues, especially flower petals and sporulates profusely under high relative humidity. Remember that blossom blight can also be caused by Colletotrichum acutatum (anthracnose), which looks a lot like Phomopsis twig blight, or Monilinia vaccinii-corymbosi (mummy berry flower strikes). With the medium-range forecast suggesting a low risk of spring frosts, Pseudomonas (bacterial) blight may not be a big issue. It has occasionally been prevalent in ‘Elliott’ fields in years with frequent spring frosts.

Phomopsis twig blight is characterized by twigs with dark brown lesions, which usually expand from the tip downwards. Since the fungus tends to enter the blossoms at the tip of the twig, these are usually the first casualties, followed by blossom clusters further down the twig as the lesion expands, which can happen fairly rapidly. Sometimes, Phomopsis moves into the twig from an infected flower bud, which never opens or starts to open and dies; we suspect these could be fall or very early spring infections.

Twig blight caused by Colletotrichum acutatum is visually indistinguishable from Phomopsis twig blight, whereas twigs infected by Pseudomonas syringae look very dark brown to almost black. Botrytis blossom blight is characterized by dead blossoms covered with fluffy grayish-brown spores. Leaves and twigs may also show spreading, light to mid-brown lesions. Mummy berry flower strikes look light brown with a dense layer of tan-gray spores on the blossom stems (pedicels). However, flower strikes are rare and only occur in the presence of shoot strikes, which tend to be much more numerous. Spring frost injury affects mostly the leaf tips, which show a dark, purplish discoloration from the tip down, but not along the midvein as in mummy berry shoot strikes.

To protect flowers from blossom blights and, at the same time, protect bees from unnecessary exposure to fungicides and adjuvants, Michigan State University Extension recommends spraying fungicides at late pink bud instead of at bloom. With any luck, you will be able to coast through the bloom period without having to put on another spray during that time. In addition, if you do a good job of preventing mummy berry shoot strikes prior to bloom, you may not need any bloom sprays for mummy berry except if you had a mummy berry-infested field nearby from which bees could bring in spores to your field. If bloom or petal fall sprays are needed, apply them at night to reduce the risk of getting spray residues on bees.

As far as which fungicides to choose, for Botrytis blossom blight, the following fungicides are effective: Captevate, Elevate, Switch, Rovral (plus spreader-sticker) and Pristine. Phomopsis twig blight infections are best prevented by Indar, Quash, Quilt Xcel, and Pristine. Anthracnose blight is best controlled by Abound, Pristine and Omega. Phosphite fungicides like Aliette, Phostrol and ProPhyt have moderately good efficacy against anthracnose and possibly also Phomopsis. In addition, Ziram, Captan and Bravo are broad-spectrum protectant fungicides with moderate to good efficacy against most blossom blight pathogens. However, Bravo may be phytotoxic to developing fruit, so use is not advised after bloom. In addition, these products may be toxic to bee larvae when carried into the hive, so use them with caution when bees are present.

Most of the newer fungicides are systemic, which is helpful during rainy periods as they tend to distribute better within/over plant tissues, be more rainfast and have limited curative (post-infection) properties. Serenade (plus NuFilm P), Double Nickel 55 and Regalia are OMRI-approved and have moderate protective activity against blossom blights. Serenade and Double Nickel 55 are strictly protectants, whereas Regalia stimulates natural plant defenses and should be applied 24 to 48 hours before infection to have an optimal effect. It should be mentioned that Serenade may increase anthracnose fruit rot in some cases – the reason for this is not known. The OMRI-listed fungicide Sil-Matrix (potassium silicate) has shown good efficacy against anthracnose fruit rot and moderate efficacy against Botrytis blossom blight.

For relative efficacy ratings, see page 243 in the “2015 Michigan Fruit Management Guide” (E0154).