Thursday, October 6, 2011

Evaluating leafspot effects

Late September is a good time of year to evaluate the effects of leaf spot diseases.  If most of your leaves are still attached to the bush at this time of year, then most likely flower bud formation has occurred and the crop potential for next year is good.  If, however, your leaves have already detached by the end of September, then chances are that some flower buds did not form due to premature defoliation, and crop potential for next year has already been reduced.

The image below shows three twigs from the cultivar Star, collected on 30 Sep 2011.  Star is susceptible to Septoria leaf spot caused by the fungus Septoria albopunctata. These are older twigs from the first flush of leafy growth in spring, and by now should have formed visible flower buds at the base of each leaf.

The shoot on the left is from a bush that was protected with a fungicide, retained its leaves, and has formed 5-6 flower buds.  The unprotected shoot in the middle defoliated early, and by comparison formed only a couple of flower buds at the tip.  The shoot on the far right was also defoliated early, not by leaf spots but by hurricane Irene.  It too, has only one or two flower buds.  Note that the shoot defoliated by hurricane winds still has the leaf petioles (bases) attached -- the leaves did not abscise normally, but were torn away:  This also happens (leaves missing, but petioles still attached) with caterpillar feeding damage. 

Regardless of the cause, when the leaves are taken away too early, flower buds do not form, and the potential crop is reduced for the following year -- because fewer flower buds means fewer berries.

The two main fungal leaf spot diseases that cause defoliation on blueberries in North Carolina are Septoria leaf spot caused by Septoria albopunctata, and Anthracnose leaf spot, caused by Gloeosporium minus. The two diseases have very different symptoms, as shown below:

Anthracnose leaf spot (Gloeosporium minus)
Anthracnose leaf spot, shown above, consists of large lesions that often occur at the edge of the leaf. Examples of susceptible cultivars are Pender (shown) and Duke.

Septoria leaf spot (Septoria albopunctata)
 Septoria leaf spot, by comparison, produces dozens of small lesions on each leaf.  The southern highbush cultivar Star (shown) is very susceptible.

By evaluating leaf drop at this time each year, growers can get a good idea of how much disease pressure they have on their farm, whether fungicide use is warranted, and which cultivars are susceptible.