Protoporphyrinogen oxidase (PPO) enzyme inhibitors are a common class of herbicides used within almonds. They include the active ingredients oxyfluorfen and carfentrazone-ethyl. These compounds are reasonably effective in managing broadleaf weeds. Due to their limited translocation within the plant, they are often referred to as contact herbicides even though they can also provide residual control at higher rates.

Off-target drift by these herbicides can damage almonds. Generally, however, the effects are more short-term due to the contact nature of these products on foliage. The reduced medium- to long-term effects of herbicide drift is useful when utilizing these chemistries on young trees. Even so, drift should be avoided as this can cause leaf spotting and loss, reducing photosynthetic area. More information on PPO herbicides and drift can be found here.

PPO herbicides have a distinctive pattern when drifted onto almond leaves. At the location where the droplet lands, syptoms occur relatively rapidly. The lesion typically has a yellow outer-halo, a purple middle-halo, and a brown lesion in the center. These lesions fall out of the leaves in 2-3 weeks, giving the appearance of shot-hole fungus.

Symptoms of carfentrazone-ethyl drift on almond leaves. Note the yellow outer-halo, with a purplish middle-halo. Within 2-3 weeks, these areas will fall out of the leaf giving the appearance of shot-hole. Photo by David Doll.
Symptoms of carfentrazone-ethyl drift on almond leaves. Note the yellow outer-halo, with a purplish middle-halo. Within 2-3 weeks, these areas will fall out of the leaf giving the appearance of shot-hole. Photo by David Doll.
This young almond tree has been drifted with oxyfluorfen. Leaf symptoms include the yellow and purple halos that encircle a brown lesion. Please note that the leaf in the lower left corner is showing symptoms of paraquat/gramoxone drift. This herbicide is different from PPO herbicides due to its more "bleached" appearance.
This young almond tree has been drifted with oxyfluorfen. Leaf symptoms include the yellow and purple halos that encircle a brown lesion. Please note that the leaf in the lower left corner is showing symptoms of paraquat/gramoxone drift. This herbicide is different from PPO herbicides due to its more “bleached” appearance. Photo by David Doll.
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