Bloom and early fruit development: A good time to control fruit rots in blueberries

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Fungal fruit rots, especially anthracnose caused by Colletotrichum acutatum, continue to be of economic concern in blueberries. Losses can occur before as well as after harvest. The cultivars Jersey, Bluecrop, Rubel, and Blueray are very susceptible to anthracnose fruit rot, whereas Elliott is quite resistant. Alternaria fruit rot may be found on Bluecrop fruit before harvest and affects most varieties after harvest (the stem scar of picked blueberries is very susceptible to infection). Botrytis fruit rot is not as common in Michigan, but may be a problem in years when cool, wet weather prevails during bloom and fruit development. These rots can be distinguished to some extent with the naked eye: anthracnose is characterized by wet, pink to orange spore masses; Alternaria fruit rot by a dark-green, velvety mold, and Botrytis by fluffy, tan-to-gray fuzzy mold on the berry surface. See the blueberry website for pictures of symptoms.

The anthracnose fungus overwinters in dead fruiting twigs, but has also been found to overwinter in live, dormant buds. The infected buds may die in the spring and support sporulation of the fungus or the opened budscales may sporulate close to the blossom and developing fruit. A twig blight, which is difficult to distinguish from Phomopsis twig blight, can also result from bud infection. With anthracnose, there are two important periods when the infection risk is high because of peak spore release: from bloom to about pea-size berry (due to overwintering inoculum), and from first blue fruit until the end of harvest (due to sporulating berries that infect surrounding berries). Fungicide spray programs should focus on these periods. For Alternaria fruit rot, aim sprays in the period between pea-size fruit and harvest.

There are several fungicide options for control of blueberry fruit rots. The old stand-by, Captan (captan) is an excellent protectant against anthracnose fruit. Ziram (ziram) has moderate-to-good activity against most fruit rots. The fungicides Abound (azoxystrobin), Cabrio (pyraclostrobin), Pristine (pyraclostrobin + boscalid), and Switch (cyprodinil and fludioxonil) are also excellent for controlling anthracnose fruit rot. Since Pristine and Switch both have two active ingredients each, they tend to have a broader spectrum of activity than Abound and Cabrio. Switch, for instance, has good-to-excellent activity against anthracnose, Alternaria fruit rot, and Botrytis fruit rot, and would be a great option if you are trying to control multiple fruit rots at once. However, both Pristine and Switch are rather expensive and should be applied during periods when multiple diseases are being controlled to make them cost-effective. The fungicide Elevate (fenhexamid) is primarily a Botryticide with suppressive activity against mummy berry. Captevate (a pre-mix of Elevate and Captan) has efficacy against anthracnose also.

All of the newer fungicides are (locally) systemic and therefore more rainfast than Captan and ziram, which are both protectants. Systemic fungicides are better options than protectants during rainy conditions, since they are not washed off readily once dry. Also, if aerial applications are made, systemic fungicides are preferable since they distribute better in the foliage and fruit. 

Dr. Schilder’s work is funded in part by MSU‘s AgBioResaerch.

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