Diseases

Avocado Root Rot

Avocado root rot.  We talk about it, we pay for research to control it, we invest in clonal rootstocks to avoid it, but are we doing our best to prevent it? I pose the question, because recently I was called out to inspect a grove I had seen a year before where I was sure the disease was rampant.  Instead of the moribund, dying grove I had previously seen, I was confronted with trees that were groaning with fruit and luxuriant canopies.

What had I seen the previous year to lead me to think that root rot was in the grove?  The characteristic symptoms are die-back, what we call stag horning.  The ends of branches are leafless and sunburned.  The canopy is thin, and the new growth is often  shriveled with a reddish burned color to the edges.  The leaves are smaller than normal and the fruit may be numerous, small and sunburned.  When digging around for roots, they are hard to find, and if located, there are few white feeder roots and they are black and brittle.  Field diagnosis is 80% certain to identify the disease, but it can only be confirmed by a lab test.  And then, only a positive test can confirm the disease.  Sampling in cooler months can often give a negative result, which does not mean that root rot is not there.

What had happened to the grove since I had last seen it?   Well, 30 inches of rainfall for one thing and a new manager, who, in spite of the $420 per acre-foot of water, had insisted that more water be applied than had previously been used.  The trees rebound due to the increased attention.  There is quite possibly root rot in the orchard, likely due to the water stress, but most of the root rot symptoms have disappeared.

Anything that compromises the root system can appear like root rot.  If the tree lacks the ability to extract water, either from lack of water or lack of roots to absorb water, the resulting above-ground symptoms look the same.  In the case of this grove, however,  there was a fine web of small roots present which did not fit into the general scheme of the disease.  Something else was causing the trees to have the appearance of disease.

Root rot-like symptoms often appear in a grove where gophers and ground squirrels have not been controlled.  Near the ends of an irrigation lateral or where pressure drops result in uneven output, trees can often develop the stag-horned look.  Trees near eucalyptus windbreaks, usually within two rows from the windbreak, often have the root rot cast, because of competition for water.

If the tree does not have root rot, it quite possibly can become diseased, if it is water-stressed.  For root rot to occur it is necessary to have the avocado tree present, the fungus, and also the conditions of stress, usually too much or too little water.  However, the stress conditions can also include such things as frost and salt damage, anything that impairs optimum growth of the tree.

The major cultural activity in the month of July is irrigation.  We are always talking about exotic sprays and fertilizer formulations, but in irrigated agriculture the most profound impact we can have on productivity is timely and adequate application of water.  To avoid the appearance of root rot and to prevent the disease it is also necessary to properly manage irrigation.  Keep reading those tensiometers!