Herbicide Drift or Fungal Disease?

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?

This one is a little bit harder, but it comes down to the coloring of the leaf tissue surrounding the lesion. Kurt Hembree, Weed Management Farm Advisor in Fresno County and Brad Hanson, UC Weed Specialist, have noted the following:  lesions caused by paraquat are localized, tan in color, and remain attached (figure 3). Carfentrazone (Shark) lesions have halos that lack color (figure 4). Oxfluorfen (Goal) lesions tend to have a defined halo, sometimes purplish and/or yellow in color (figure 5).  Both of these herbicides will cause lesions to fall from the leaf.

Figure 3: Shot Hole like damage caused by  Gramoxone (Paraquat). Note that the lesions do not fall from the leaf

To help with the diagnosis, observe the location of the damage. Is it over the whole tree, or just lower few branches? Is it just on the outer rows? Is it across both varieties? What is the history of herbicide use?

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).