Keeping Blueberries Pest-Free

With the headlines blueberries have been getting in recent years, thanks to their health benefits, it stands to reason that growers need to keep up with effective blueberry crop protection strategies. This year’s winter meetings have been filled with discussions on this topic. This reportcovers two talks presented at meetings this past year. The first section, based on a presentation at the Southeast Regional Fruit and Vegetable Convention by North Carolina State University plant pathologist Bill Cline, covers the emergence of a potentially damaging disease: Exobasidium, or green leaf spot. The second section offers a review of new weed control options for blueberry growers, and it is based on a presentation given at the Great Lakes Fruit, Vegetable & Farm Market Expo by Michigan State University’s Eric Hansen.

Exobasidium And Leaf Spot

Symptoms caused by the fungus Exobasidium vaccinii have occurred sporadically in individual blueberry fields for years, but have not posed an industry-wide threat to blueberry production. However, in 2011 growers in North Carolina saw increased incidence of this disease, and symptoms on previously unaffected cultivars such as Legacy and Columbus. The fungus causes spots on both leaves and berries. Spots on fruit are especially problematic because it is impossible for pickers to avoid harvesting affected berries, and nearly impossible for color-sorters and packingline inspectors to remove them during the sorting and packing process.

On berries, infection produces a green spot that fails to ripen normally. Affected berries do not leak or decay, but the green spot on an otherwise uniformly ripe berry is an unsightly defect that could lead buyers to reject the fruit when delivered. On leaves, spots are pale-green on the upper surface but pure white below, with a thin, dense layer of fungal growth on the underside of the leaf. This fungal growth is most obvious on the underside of leaves, but can also occur on infected berries.

As spots on leaves age, they become brown and necrotic, although the white fungal layer is often still visible on the underside of the leaf. Eventually, the unique symptoms fade and the spots become indistinguishable from many other leaf-infecting fungi.

The fungus produces spores on both leaves and berries. They are often a distinctive “musiform” or banana shape, and may be divided (septate).

Little is known about the life cycle of this fungus on blueberry. Infections appear in the spring on developing leaves and berries, but the fungus does not appear to infect later flushes of leaf growth. Lesions have not been observed on other plant parts (stems, buds) and it is not known how this pathogen overwinters. Visually, infections appear to be localized, distinct, and limited to the affected berry or leaf, rather than systemic in the plant. Since blueberries drop their leaves each winter, there may be some quiescent infection stage on or in the remaining, bare dormant stems, or buds that serves as the overwintering mechanism.

Fungicides have been shown to be at least partially effective in controlling this disease. A study by David Ingram and John Braswell at Mississippi State University achieved measurable control on rabbiteye blueberries. The combination of pyraclostrobin and boscalid (Pristine, BASF) was most effective in their tests.

Why is this disease becoming more prevalent? It may be due to changes in the cultivars we grow, the loss of key fungicides in recent years, or changes in cultural practices such as the increased use of irrigation. This disease appears to be an emerging problem not only in North Carolina, but in other southern states as well. The more we know about it, the sooner we will learn how to manage it.

For information on new herbicides for blueberries, go to the next page.

This past year brought both bad news to blueberry growers (increased disease pressure) and good news (new options for weed control).