Bloom is around the corner. After receiving adequate chill units through December (almonds need ~400-500), almonds switch from ‘chill accumulation’ mode to ‘heat accumulation mode.’ Although less is known about this stage for many crops, it is known that a certain amount of heat is needed in order for bloom to begin. This heat requirement keeps the plant from blooming too early, reducing the risk for the impacts of frost events. The warmer weather over the past few weeks, however, has provided conditions that have met the heat requirement for most areas of CA. This suggests that if the weather stays warm through the next week, bloom will begin for more areas within California. Below are some considerations to have in mind for bloom:
Fungicide sprays. Due to the sensitivity of almond flowers to disease and conditions during bloom that are conducive for disease, most people will spray a fungicide during this period. In wet conditions, multiple sprays may be needed, while in dry years a single or no spray may suffice. Conditions that favor disease formation include wet weather, warmer temperatures for Brown Rot, Anthracnose, Bacterial Spot, and Shot-hole, cool weather for Botrytis, and heavy dew. Fungicide selection should provide coverage for the diseases of concern (Please see this link and scroll to ‘Almonds’). Bloom sprays should be applied before rain events to provide protection for flowers, flower parts, and emerging leaf tissue.
Coverage from a fungicide spray will last around two weeks, unless significant rainfall occurs. If applying prior to a rain event, applications will need a few hours to dry to prevent “run-off.” In rainy weather, follow-up sprays will be needed every 7-10 days. Some varieties – such as ‘Butte’ and ‘Carmel’- are more susceptible to disease and may require a spray even in dry weather, while others – such as ‘Nonpareil’ are quite tolerant and may not need a bloom spray. Orchard history, weather, and your comfort level should be the guidance in determining your bloom and springtime disease control strategy.
Fungicide rotations. It is important to avoid back-to-back application of fungicides within the same mode of action. Mode of actions is simplified using the FRAC number – and thus back-to-back applications of the same FRAC number should be avoided. This includes pre-mixed fungicides. An example of a rotation program for a multiple sprays for rainy weather include: First Spray: FRAC 9 (Scala, Vanguard), 2nd Spray: FRAC 11 or FRAC 7/11(Gem, Abound, Pristine, Luna Sensation, Merivon, etc), 3rd Spray: FRAC M4 (Chlorothalonil), 4th spray: FRAC 3, Frac 11, or FRAC3/11 (Bumper, Tilt, Gem, Abound, Indar, Quadris Top, Quilt Xcel, etc). Note how the two applications of FRAC 11 were split by rotating away to another chemistry. Keep in mind that FRAC 3 (DMIs) do not provide any protection for Botrytis. More information on fungicide selection can be found here. Please note that the fungicides listed are an example, not an endorsement for use. Please refer to Timing and Efficacy Tables for full list of tested fungicides. There are many effective fungicides not listed in this example.
Insecticide applications at bloom for peach twig borer (PTB). Recent research has found that applications of diflubenzeron timed at bloom has a negative impact on bee health. The insecticide has been shown to reduce the survival of immature queens. It is unknown if other insecticides often tank mixed in at bloom create the same effect and until more research is conducted, it is recommended that insecticide applications at bloom be removed. Other timings for PTB include the ‘May Spray’ (which may reduce NOW populations) and dormant. Please see the UC IPM page for PTB for more information.
Early bloom and frost risk. The past two years, bloom has been earlier than normal. This increase the risk for frost and freeze damage. Make sure that annual maintenance of the irrigation system, pumps, and filters has been completed in preparation for a ‘freeze event.’ Even though the coldest part of the nigh occurs just before sun-up, water applications – which release heat – must be operational before the temperature drops below the critical threshold. Here is a link to an earlier article describing practices to reduce frost/freeze damage.
Pre-emergent herbicide applications. If applying a pre-emergent product after the trees become active (~early February) it may be of benefit to reduce the rate of pre-emergent to the lower end of the label rate to reduce the risk of crop injury. This is of more concern with low cation exchange capacity soils and in years with late incorporating rains.