Wednesday, October 30, 2013

Leaf scorch caused by pecan anthracnose

   This fall you might have noticed pecan leaves with brown, crispy margins (photo at right). This past summer's rainy weather helped to promote the spread of this disease once known as "fungal leaf scorch". Now, during the month of October, this disease has caused the early defoliation of some pecan trees.
    Leaf scorch is actually just one symptoms of a late-season disease called anthracnose which attacks both pecan leaves and nuts. Pecan anthracnose is caused by the fungus, Glomerella cingulata, a common plant pathogen that causes diseases in many fruit and vegetable crops.
   On pecan leaves, anthracnose first appears as brown, irregularly shaped lesions along the edges of a leaflet. These lesions can spread rapidly over the entire leaflet ultimately causing early leaf fall. The advancing margin of the infection forms a distinctive dark brown line that separates healthy tissues from disease killed tissue (photo above).      

    Anthracnose infection of the nut can start as small sunken lesions on the shuck but the disease can spread to cover a large part if not the entire shuck (photo at left). Infections that cover the entire shuck by early August (water stage) can cause smaller nut size and prevent normal shuck split. Anthracnose infections that occur late in season seem to have little impact on the nuts other than to advance the shuck opening process.

   To determine the influence of anthracnose on nut quality, I collected nuts with infected shucks, peeled them out, then cut them in half to check kernel fill. The nuts that appear in the photo at right represent a range of anthracnose disease severity--from less disease on the left to the most severe on the right. The nut that came out the the shuck appears directly below the nut-in-shuck photo. Below each nut is that same nut cut in cross-section.
    The first thing I notice in the photo is that all 4 nuts are well filled with kernel. Looks like this year's anthracnose infection on nuts will have little effect on kernel quality. However, the nut on the far right was smaller than the others and the shuck did not peel off very easily. In this case, anthracnose probably got an earlier start on colonizing this particular nut.
    Even though the kernel inside the nut on the far right looks good, this nut would probably not survive the nut cleaning process. Judging from the force I needed to apply to remove the shuck, this nut would probably be discarded off the cleaning table as a stick-tight. If anthracnose causes enough stick-tights, this disease can lead to significant yield losses.
    Anthracnose can be controlled with fungicides and is largely suppressed when we spray fungicides to control pecan scab. However, this year's unusual weather patterns hit just right to promote an outbreak of anthracnose after we finished making fungicide applications aimed at controlling scab.