In the last two weeks I’ve been out to see groves that have root rot, yet the growers did not recognize the signs. Two years of drought and use of salt loaded water have put stress on trees and made them more susceptible to root rot. Although irrigation management can bring about the symptoms of root rot – small, yellow, tip-burned leaves – because the tree in both cases is seeing a lack of water. I thought it would be appropriate to review the symptoms of avocado root rot.
What to look for:
Small, yellow, tip-burned leaves
Die-back in the canopy, causing stag-horning (dead stems)
Little or no new leaf growth, hardened look to the leaves
Few or no leaves on the ground (tree doesn’t have energy to produce leaves)
Profuse flowering and small fruit
Sunburned fruit from reduced canopy
Then get on your knees and dig around in the wetted area of the root zone
Do you find roots in the top 3 inches of soil? NO, that’s a bad sign
Do you find any white root tips (it’s hard to find these when the soil is cold in the winter)? NO, that’s a bad sign.
Do you find black roots? Yes, that’s a bad sign.
These are all field diagnostics for avocado root rot. You can also sample roots and send them in for lab analysis, but in the winter, the root rot organisms are not active and you can actually get a false negative. Meaning the lab won’t pick it up and you then think you don’t have the disease. Use field clues to figure out whether you have root rot. Then figure out why you have it. It usually boils down to the amount and timing of water, but there are many other factors, such as water quality, fruit load, topworking and other stresses that can bring on the disease.
Images. Root rot in canopy, leaves and roots
Iron chlorosis is an off duck. Soil is basically iron with some aluminum, silicon and oxygen added in for good measure, but plants can lack for iron because it is not available to be taken up. This is often because the soil pH is high due to carbonates which tie up the iron, causing it to precipitate. Another cause is asphyxiation of the roots from too much water. Avocados are especially sensitive to asphyxiation and will show classic interveinal chlorosis and as time goes on, the leaf will turn almost white. They will also show tip burn, like chloride damage in some cases, along with the yellowing. The image of the tree below is of an avocado in a loamy soil with a pH near 7, but is irrigated every two weeks with about 400 gallons of water – kind of boom and bust. The trees are drowning and showing classic iron chlorosis symptoms, but they could also be confused with root rot.
Amazing. I was out with the contractor who has been doing the California Avocado Commission's annual acreage report. In the past, aerial photography was used and with painstaking accuracy the acreage was visually evaluated by hand. It was so expensive and took so long, that only parts of the California acreage were done each year. The company now uses satellite imagery and computer evaluation to do the whole avocado growing area each year and at much less cost. Now the contractor is going to try to estimate the amount of root rotted acreage that is out there. They will do this by canopy color and texture relative to healthy trees. We were out looking at the range of diseases out there that could be confused with root rot, such as bacterial canker, blight, black streak and crown rot. This will be amazing if they can distinguish amongst the diseases, but even if they can identify unhealthy groves that will be an amazing feat.
Armillaria Root Rot
There have been a lot of new avocado orchards planted during the last few years. These often have been in old ‘Valencia’ orchards or lemons that had poor production. In order to save money, growers have just cut the trees at ground level and replanted the avocados near the stumps. Avocados have recognition of being resistant to Armillaria, but in this environment of high disease pressure, they can fail.
Armillaria root rot is common, yet is an infrequently identified and poorly understood disease. It is capable of attacking most species of trees and other woody plants growing in California. It is sometimes called “shoestring root rot” and the causal fungus is often referred to as the “honey mushroom.” Because oak is one of the preferred hosts, it is alsocalled “oak root fungus.”
If a tree undergoes a slow to rapid decline without any obvious reason, suspect Armillaria as the cause. Certain areas, such as drainage areas from chaparral or woodlands are likely areas for this disease. Old roots left underground provide a food base for continued fungal growth and survival.
General symptoms of Armillaria resemble those of other root disorders. These symptoms are disrupted growth, yellow foliage, branch dieback, and resin or gum exudates at the root collar. Trees may die rather abruptly without showing any decline symptoms. Avocados typically have a rather protracted death, but in citrus it can be rapid. Only rarely can the disease be diagnosed without examining the larger buttress roots and root collar of the tree. After carefully removing the soil, examine forthe presence of:
1) Rhizomorphs, or fungal ‘shoestrings’ attached to the wood under the bark. These may occur beneath the bark for some distance above the soil line in advanced cases, rarely they may radiate from the wood into the soil. Rhizomorphs may also grow out from the larger roots, resembling feeder roots in appearance. They are about the diameter of pencil lead and vary in color from black to reddish brown. The interior consists of white mycelial tissue.
2) Decayed areas of wood at the root collar or on the crown roots. Armillaria causes a white rot and the wood develops a stringy texture. Roots in advanced stages of decay may be soft, yellowish and wet.
3) Veined, white mycelial fans between the bark and wood where the cambium has been killed. Sometimes this fan (or fans) extends quite far above the soil line beneath the bark.
4) Soil remaining attached to theroots.
5) Characteristic mushrooms on the lower trunk or on the ground near the infected roots. These shortlived annual fruiting structures of the disease-causing fungus may develop during the fall or winter rainy season and may occur in small clusters or in large numbers. The stalk is typically yellow and 3 inches or more long. Usually a ring is connected to the stalk just below the cap. The cap is 2-5 inches across and often honey-yellow. It may be dotted with dark brown scales. The underside is covered with loosely spaced white or yellow gills radiating from the stem. After the disease has been identified, the grower should study the situation to determine the role Armillaria root rot has played in causing the decline or death of the tree. Frequently the fungus is only involved in a secondary manner by invading and destroying roots after the tree has been exposed to stress of some form, such as severe drought, water logging, or soil fill over the roots. The fungus can also act as a saprophyte feeding on dead wood. It is frequently involved in the decay of old tree stumps and roots.
Many oaks are lightly infected with the disease for years with no resultant damage except for isolated pockets of buttress root rot which are walled off by the tree and have no ill effects. Other infected trees show no damage until subjected to stress. Accumulating evidence suggests the type of root exudate that is produced influences the susceptibility of the tree. Certain forms of stress cause a shift in exudates that promote rapid development of the fungus and may hasten tree invasion and decay.
Spores are produced by the mushroom fruiting structures (mushrooms) and disseminated by air currents and introduced into new area. Once the fungus enters the cambium and bark tissues, mycelial fans develop during the parasitic phase of the attack. Subsequently, mycelium invades and decays the woody tissue of the roots and sometimes also the base of the trunk. Under proper conditions the fruiting structures form at or near the base of the infected tree, completing the life cycle.
Direct control of the fungus in a diseased tree is not possible with present technology. However, in many instances the fungus is incapable of causing severe damage unless the tree is first subjected to substantial stress. Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing loss of trees to Armillaria.
Drought and leaf defoliation are two major forms of stress that favor Armillaria. In dry years it is advisable, as in all years, to make sure irrigation scheduling is appropriate. Stresses such as defoliation from persea mite, soil compaction and physical injury can exacerbate the disease. Nutrient management may minimize Armillaria effects, although little research information exists on this subject.
The second most important means of minimizing Armillaria damage is to avoid or eliminate the fungus inoculum before planting. Trees planted in former orchards will quite possibly be exposed. Since these sites cannot be avoided, here is a suggestion that will be helpful: remove stumps and old roots from the old orchard to the greatest extent possible.
South African plant pathologists were the first to show that root rot in avocado could be controlled by trunk injection with both phosphorous acid and the patented material Aliette®. Aliette was briefly registered in California in the late 1980’s, but theregistrant soon lost interest in pursuing a full pesticide registration when it became apparent that other researchers believed phosphorous acid could be registered as a fertilizer - a process much less costly and simpler than a pesticide registration. The company continued to hold on to the patents for the product and the breakdown products that were useful in root rot control. By holding onto the patent, this effectively stopped other companies from pursuing a pesticide registration for phosphorous acid. In 1990, a publication reported that phosphite could be used as a source of phosphorus fertilizer and this became the basis for the registration of phosphite as a fertilizer. Subsequently, when the original patent expired, at least two materials have been registered as fungicides containing phosphite – Fosphite® and Agri-fos®. There are, however, numerous phosphite materials that have been registered as fertilizers (for some brands see Brunings et. al., 2005, http://edis.ifas.ufl.edu/HS254), and every day seems to bring more brands onto the scene each making claims of having the best efficacy.
We wanted to see if we could detect an efficacy difference between Aliette, another registered phosphite fungicide and four different materials registered as fertilizers, for a total of six materials. In a greenhouse, three-month old ‘Topa Topa’ seedling avocados with cotyledons removed were planted into a Phytophthora cinnamomi -inoculated organic potting mix. A control was also planted without the inoculum, as well as an inoculated control. One of six different materials was then applied as a soil drench until draining from the bottom of the liner. The materials were applied at the equivalent phosphorous acid concentration. There were 20 replicates for each of the controls and treatments. The experiment was repeated twice.
At harvest, root fresh and dry weights were highest for the non-inoculated trees and lowest for the untreated, inoculated controls, in both trials. All treatments’ associated weights intermediate between these two were statistically the same. Even a repeat application of one of the treatment materials in trial II didn’t result in greater root weights than single application treatments. Shoot weight, both dry and fresh, was much less affected by root rot and treatments. There were no differences in fresh shoot weight in the second trial, not even between the inoculated and noninoculated controls. The root and shoot weights of all the treatments in the second trial were higher than in the first trial, indicating that either the inoculum was not as effective or that the trial was not continued long enough to produce as much damage.
Root rot studies often have dramatic effects on root weights while shoot weights may remain little affected. It is clear from our data that phosphites reduced the severity of root rot in this study, but that there was no benefit of a single source of phosphite relative to any other source.
Below: Healthy and decaying avocado roots.