Figure 1: What caused this damage:
Herbicide drift or fungal disease?
Leaf samples often appear on my desk with notes asking to identify the problem. More often than not, symptoms appear shothole-ish in nature, and growers are questioning their fungicide program (Figure 1). These symptoms, however, also appear from drifting herbicides. How can one tell the difference between three common herbicides and the fungal disease shot hole?
Answer: look for fungal fruiting bodies. Shot hole infected leaves can be determined by the black fruiting body that is centrally located in the lesion on the top side of the leaf (Figure 2).
|Figure 2: Shot Hole affected
almond leaf. Note the black
fruiting body in the center
of the lesions.
What about telling the difference between the three herbicides?
Figure 3: Shot Hole like damage caused by
Gramoxone (Paraquat). Note that the
lesions do not fall from the leaf
Herbicide damage tends to be found on the lower portion of the tree, with damage on all varieties within the orchard. Drift damage also tends to increase on the outer few rows as winds are stronger within that orchard position, moving herbicide particles into the first few rows of trees. Shothole damage should appear all over the tree – from top to bottom. It may not appear as severe across varieties because of differing susceptibility.
|Figure 4: Shot Hole like Damage caused
by Carfentrazone (Shark).
|Figure 5: Shot Hole like symptoms caused
by Oxyfluorfen (Goal).