|Green spots caused by Exobasidium on 'Legacy', 1 June 2011|
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.
|Exobasidium leaf spot -- upper surface|
|Exobasidium leaf spot -- underside of leaves showing fungal growth|
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.
|Exobasidium on 'Legacy' leaves and fruit 7 Jun 2011|
Eventually, the unique symptoms fade and the spots become indistinguishable from many other leaf-infecting fungi.
|Symptoms on 'Legacy' upper leaf surface 20 Jul 2011|
|Symptoms on the lower leaf surface 20 Jul 2011|
The fungus produces spores on both leaves and berries. As shown below, they are often a distinctive 'musiform' or banana shape, may be divided (septate), and measure roughly 4.0 to 5.2 μm wide × 13 to 15 μm in length.
|Drawing of characteristic spores as seen under a microscope|
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 + boscalid (Pristine) was most effective in their tests.
|Infections on the rabbiteye cultivar Premier, 13 Jun 2008|
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 to changes in cultural practices such as the increased use of irrigation. If you are experiencing problems with this disease, I would like to know about it. It 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 to manage it -- thanks for your help!
W.O. Cline, 1998. An Exobasidium disease of fruit and leaves of highbush blueberry.