Thursday, July 21, 2011

Leaf Disease Symptoms

Leaf spot diseases caused by fungi are apparent in some fields this week.  Septoria leaf spot is a disease caused by the fungus Septoria albopunctata.  Infection results in small circular spots with dead gray centers. Often fungal fruiting bodies are visible as tiny black specks in the dead gray area at the center of each spot, as in the photo below. Spots first appear on oldest leaves near the ground. The cultivar Star is very susceptible

Septoria leaf spot on 'Star'

Another leaf symptom that is evident this week (21jul2011) is premature drop of yellowing older leaves.  The cause is unknown but is suspected to be a form of blueberry leaf rust.  Rust fungi cause yellowing and premature drop of leaves, and a characteristic symptom is the appearance of green spots on otherwise yellow leaves, consistent with the image below.  This symptom is especially noticeable on the southern highbush cultivar Legacy and on the rabbiteye Premier.  Rust diseases get their name from the appearance of orange spore masses on the underside of infected leaves, but so far we have not seen spore production on these yellowed leaves.  When viewed from the underside of the leaf, the symptoms resemble edema, so there may be some environmental stress, perhaps related to the drought, that is causing or contributing to the problem.

Yellowing and premature drop of 'Legacy' leaves on 20 Jul 2011

Leaf drop is a serious problem because it can severely reduce yield for the coming year. Twigs that defoliate prematurely will not set flower buds, and so will not bear fruit the following year.  There are several fungicides that can be used to control leaf diseases;  also, summer pruning immediately after harvest removes old infected leaves and forces clean new growth, greatly reducing the effects of leaf diseases.