IN YOUR ORCHARD
and reach high enough populations to cause the canker and blast symptoms we observed this spring. Several farm advisors and myself are working with Dr. Florent Trouillas to more fully understand bud colonization in the winter by Pseudomondas syringe . Don’t get caught by surprise with freezing temperatures in April! I remember a damaging frost on Easter Sunday (mid-April) more than a decade ago. Young nuts are very susceptible to freezing conditions. As almond flowers bloom and small nuts form, they are more susceptible to frost damage. The amount of damage is dependent on the lowest temperatures received and the duration of the freeze. In the small nut stage, Nonpareil can have 25% loss with 29°F for 30 minutes, 50% loss with 28°F for 30 minutes, and 100% loss with 27°F or less for 30 minutes. A one-degree variation in temperature can mean the difference between slight damage and total loss. Irrigation and mowing down winter weeds can increase orchard temperatures. Freezing temperatures followed by warm weather can cause more damage than a frost followed by cold weather as the trees can acclimate to the cold. Irrigating dry soil a few days prior to a frost may cool daytime temperatures and increase heat storage in the
soil, enhancing frost protection. See Chapter 23 in our Almond Production Manual, University of California publication 3364 available in our county offices and at anrcatalog.ucanr.edu. A pressure chamber is especially useful in early spring to determine tree water status and when irrigation should begin. On a wet-year growers can often skip an irrigation or two because of the added water in the soil profile. Unfortunately, most growers are starting this season with a dry profile. Dr. Bruce Lampinen, our Almond Specialist at UC Davis, has written a nice article ”Using Midday Stem Water Potential to Refine Irrigation Scheduling in Almond” that can be found at the Fruit and Nut Research and Information Center fruitsandnuts.ucdavis.edu. This document provides baseline stem water potential values for fully watered trees at various temperatures and % relative humidity. For example, if the temperature is 80°F and the relative humidity is 20%, the baseline would be -7.5 bars. You should allow the midday stem water potential to fall one to two bars below the baseline (-9.5 to -10.5 bars) before beginning irrigation to assure that you are not over-irrigating. The pressure chamber will also allow you to more accurately monitor tree stress in the absence of adequate water.
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