As this year’s almond crop approaches hullsplit, it’s time to think about hull rot prevention. Effective hull rot management takes an integrated approach that includes strategic deficit irrigation (SDI) at the onset and during hullsplit, and a balanced fertility program that avoids applying nitrogen in excess of tree and crop demands.
The goal of a hullsplit SDI program, which can reduce hull rot 60% to 90%, is to track tree water status with a pressure chamber and irrigate to maintain tree stress levels between –14 bars and –18 bars from the onset of hullsplit (also coincidental with kernel fill) until 90% hullsplit, typically a period of about two weeks.
Valuable Tool
During this time, a pressure chamber is a valuable tool to track stem water potential (SWP). Even so, it can be difficult to fine-tune the irrigation schedule. Depending on the soil type, reaching this stress level can take just a few days or much longer, so timing is a challenge.
A pressure chamber is a valuable tool to track stem water potential in a strategic deficit irrigation (SDI) program to reduce hull rot and save water.
Many growers initially reduce water applications by 50% around mid-June and adjust the amount of subsequent irrigations once stress levels increase and soil moisture depletion occurs. But this will vary; for example, in shallow soils where trees may dry down quickly, stress can be initiated when blanks start to split, usually about a week before the onset of hullsplit. On deeper, well-drained soils, it can take up to 20 to 30 days to reach mild to moderate stress levels. Pressure chamber readings should be done in concert with soil moisture monitoring to ensure deep moisture is not depleted, which would impose severe stress that is difficult for trees to recover from.
In addition to reducing hull rot, hullsplit SDI can reduce water use during this period by as much as 34% and reduce the total season’s water budget by 10–15%.
At the end of the SDI period, be sure to apply sufficient water just prior to harvest to finish hullsplit, reduce hull tights and improve harvestability.
A valuable guide to implementing hullsplit SDI and how to get more information on pressure chambers is UC ANR Publication 8515, “Drought Tips: Drought Management for California Almonds,” which can be obtained at http://anrcatalog.ucanr.edu.
Fungicides
Under high-pressure situations, research conducted by plant pathologist Dr. Jim Adaskaveg, UC Davis, shows that a number of registered fungicides applied at early hullsplit combined with navel orangeworm sprays can reduce hull rot caused by the bread mold Rhizopus stolonifer about 50%. Fungicides applied at this timing are not effective against hull rot caused by Monilinia spp. Research is ongoing to determine if fungicide treatments can be an effective tool for hull rot caused by Monilinia spp. For Monilinia hull rot, applications should be done in early to mid-June. Effective fungicides for both are in FRAC groups 3+7, 3+9, 7+11, 3+11 and 3+19.
