- Author: Ben Faber
No matter where they grow in California, June is a month when avocados are being watered on a regular schedule. How regular that schedule is should be carefully reviewed by the irrigator. In 1991-'92, right along the coast in a Ventura irrigation plot, we applied 32" of water, but in '92-'93 we put on only 26". Same trees, nearly the same size, but a 23% difference in applied amounts dictated by differences in water demand due to different weather. The irrigation schedule we use is driven by tensiometers and a CIMIS weather station. The station generates reference evapotranspiration values which tell us how much water to apply at an irrigation, and the tensiometers are used to verify whether the trees are doing well by the schedule. Irrigation on a fixed schedule, such as once a week for 24 hours, is going to guarantee that on average you will be either under or over irrigating at each irrigation. Using some soil-based measure, such as a soil probe or tensiometer can assure an irrigator that trees are getting the appropriate amount of water when they need it. If you haven't done so, the irrigation calculator available at the Avovcadosource.com website can be quite useful in guiding an irrigation schedule - http://www.avocadosource.com/tools/IrrigationCalculator.asp - check it out. You also need to correct for salinity accumulation.
In orchards which have not closed canopy yet, weeding is an ongoing activity. In a research plot, we are using tensiometers to monitor the effects of weeds, bare soil and chipped yard waste mulch around trees. In weedy plots soil moisture profile rapidly show 30-40 cbars of tension at 6", whereas the bare and mulched plots can go much longer before showing 40 cbars. Centibars is a measure of moisture tension, the higher the value, the drier the soil. As trees get older they make their own leaf mulch and shade which limit weed growth. There is no question that a cover crop can improve soil conditions through reduced erosion, improved water infiltration, and possible reduced disease and pest problems. These soil improvements tend to improve tree growth and orchard productivity. But, if water is the primary issue, weeds and a cover crop can add considerably to water use in an orchard, especially a young one. Weed control through the use of mulches and herbicides can effectively reduce the water requirements of trees.
June is still a good time to replant an orchard. The soils are warm enough to give the trees a good start and there is enough fine weather left for them to establish before winter comes. Late plantings (September, October) are discouraged because the root-shocked plant sits in a cold, wet soil through the winter and becomes a prime candidate for root diseases. Especially in a replant situation, it is a good idea to start them off with a fungicide with one of the phosphonate products, to give them some protection until they get established. The best time for to apply the material to do its job on older trees is when there is a good root flush of growth which occurs after the leaf flush in spring and fall.
When replanting, try as much as possible to avoid interplanting between older trees. The different water requirements of the young and old trees is such that one or both will be stressed because they need different schedules - less but more frequent for the young trees. Attempts can be made to put together a system where the older trees remain on the 10 or 15 gpm mircosprinkler and the young trees are put on a 1 gpm dripper. This still cannot be an ideal situation, since the needs for application frequency are still different between the small and big trees. The best thing to do is to clear out trees within an irrigation block and replant, or replumb a block with a new valve so that small new block can be irrigated differently from the older trees. Where clonal rootstocks fail in a root rot replant situation, it is invariably where water control is lacking or poor.
As we all know, this has been a long dry spell in the avocado growing areas along the coast. With the levels of salt in our waters, it's important to have some kind of a leaching program to ensure that salts do not accumulate in the root zone. Each winter, rain leaches the accumulated salts from the previous irrigation season and starts the orchard off to a good start. These years it hasn't happened and one of the things that can affect the trees is a stress. This is a salt stress that is most pronounced at the end of irrigation lines and where low pressure results in low output, often at the top of the hill. Any irrigation system that has poor distribution uniformity is going to have areas where less water than average is applied.
One of the responses of the trees to salt stress is to exhibit cankers in the branches. These can be silver dollar-sized cankers running in a line up the branch or as diffuse white spots in the branches. The first symptom is related to bacterial canker and the second is to black streak. These are not killer diseases, but they are reflective of the tree being under stress. As soon as the irrigation schedule is corrected, these symptoms can clear up in several months. If the schedule is not corrected the tress will begin losing leaves and sunburn can result. The symptoms of these two problems can be viewed at the UC Integrated Pest Management website - http://www.ipm.ucdavis.edu/PMG/r8100611.html and http://www.ipm.ucdavis.edu/PMG/r8100311.html.
Again these are primarily stress-related diseases and the way to correct the situation is to improve irrigation distribution uniformity and the irrigation schedule. If it goes on too long it can cause problems especially in young trees. When you boil down farming to the basics, the most important activity in the orchard is ensuring proper irrigation.