Friday, July 29, 2011

Exobasidium fruit and leaf spot

Symptoms caused by the fungus Exobasidium vaccinii have occurred sporadically in individual fields for years, but have not posed an industry-wide threat to blueberry production in North Carolina.  However, in 2011 we 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 packing-line inspectors to remove them during the sorting and packing process.

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.