UC Cooperative Extension Ventura County
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Wet Weather Brings Pluses and Problems - 1995
The large amount of rain we have been having this season brings good news and potentially bad. The rain leaches out accumulated salts that we have placed in the root system by irrigating during the previous summer and fall. It cleans the leaves of dust and reduces mite populations, even the persea mite's. It waters the whole orchard, and not just the wetted area of the irrigation system, making more nutrients available to the tree. It also gives a respite from paying costly water bills.
On the other hand the rains we have had will pose problems of disease and physiological problems for the trees. When we get warm tropical rains, they bring the potential for widespread fungal problems, such as anthracnose and dothiorella. These can cause fruit decay, as well as uneven, dead spots on leaves that can lead to defoliation. These two fungi are normally in orchards, but are only set off in mild, wet winters. Their infections are latent on the fruit, normally appearing as black, uneven spots when the fruit ripens. In the case of the Phytophthoras, cinnamomi and citricola, there can be readily noticed lesions that form on the neck or the base of the fruit. These spots form wherever there has been rainsplash from the ground, sending the soil-borne spores onto the fruit. Subsequent splashing of lower infected fruit can stairstep the infection up onto higher fruit. There are no chemical controls for these diseases. In most years these diseases are not a problem in California. In Mexico and Florida, copper sprays are applied as a preventative in anticipation of rain. Cultural practices, such as skirt pruning to raise fruit above the ground and canopy pruning to improve air circulation can reduce the incidence of these diseases.
Wet winters with their potential for runoff and erosion, can also spread avocado root rot. Aside from the costly impacts of rebuilding roads, loss of trees and loss of fertile topsoil, there is the cost of the long term health of the orchard by the increased potential for root rot's spread. Anticipate erosion problems by proper road construction, and make sure preventive maintenance is made on drains, check dams, and other erosion control structures.
Wet soil can also have the more immediate impact of asphyxiation. This condition arises when trees in waterlogged soils are suddenly put into a high water demand situation - the sun comes out, the wind picks up and the temperature rises. Asphyxiation is more common later in the winter or early spring, but can occur anytime roots have been in a prolonged wet condition, such as when a broken water line soaks the soil, and sudden evaporative demand is applied.
The appearance of asphyxiation is the sudden collapse of the tree, almost like frost damage. In fact, it has been confused with frost damage, because of the rapidity of onset. The rain clears up, the sun comes out and the once-green trees are ghostly brown in a few days. This can happen on flat ground and even on seemingly well-drained ground of fairly steep slopes. It is a most forlorn condition, but fortunately, especially older trees that have reserves, the trees grow out of it. Nothing should be done to the trees until they indicate where new growth will occur, then prune if necessary. If this condition occurs later on in the year, whitewashing is recommended for sun protection.
Although the rains so far have not seemed to depress the population of the avocado thrips pest and may bring disease problems, we should view these rains as a plus. The rain also gives us the opportunity to switch over from drip to microsprinklers or some other sprinkler pattern. By switching at this time, the trees can take advantage of the increased soil moisture to adapt to the change.