Hull rot disease is caused by two pathogens that thrive on split hulls of well-watered and well-fertilized trees. The best defense against these pathogens is to cut back on irrigations and check that nitrogen rates do not exceed crop demand.
Hullsplit strategic deficit irrigation (SDI), applied from post-kernel fill through 90% hullsplit, a period of about two weeks, can reduce hull rot by 60 to 90%. This is accomplished by tracking tree water status with a pressure chamber, and irrigating to maintain tree stress levels (stem water potential) between -14 bars and -18 bars during the hullsplit period, depending on the weather.
The duration of the irrigation should be reduced, rather than the frequency, to meet the desired rate of application.
SDI can also benefit long-term orchard health by inducing an even hullsplit, which reduces the force and time required for shaking during harvest, and assists with navel orangeworm control.
Many growers initially reduce applied water by 50% about mid-June, and adjust the amount of subsequent irrigations once stress levels increase and soil moisture depletion occurs. This will vary, however, depending on soil characteristics. In shallow soils, in which trees may dry down quickly, stress can be initiated when blanks start to split, usually about a week before the onset of hullsplit.
Role of Nitrogen
Long-term studies performed by University of California researchers found a strong relationship between nitrogen (N) rates and hull rot. To avoid excess N, a leaf tissue analysis of nitrogen content should be conducted to determine N status, and nitrogen rates should be adjusted, according to crop load as specified in nitrogen budgeting protocols outlined on the Almond Board website.
Research suggests that summer nitrogen applications increase hull rot incidence; N that is applied after kernel development goes to the hull, increasing the chances of infection.
The two primary pathogens causing hull rot, Rhizopus stolonifer and Monilinia fructicola, both infect fruit and can cause spur and limb dieback, potentially reducing future crops.
When it comes to treatments to help manage hull rot in high-pressure situations, the two pathogens require different management strategies. As these pathogens grow, they produce toxins. For instance, as R. stolonifer grows, it produces fumaric acid, which is responsible for dieback.
Plant pathologist Dr. Jim Adaskaveg, UC Riverside, has found that foliar alkaline treatments with potassium phosphate fertilizers have been effective in reducing hull rot, possibly by neutralizing fumaric acid.
In addition, fungicides can reduce the incidence of disease, but different timings are needed for the two pathogens:
- For Monilinia hull rot, fungicides should be applied in late spring, typically late May to early June; and
- For Rhizopus hull rot, applications should be made at early hullsplit along with navel orangeworm applications.
Treatments evaluated in Dr. Adaskaveg’s trials that significantly reduced the disease as compared to the control were fungicides in FRAC Groups 19, 3+7, 7+11, 3+11 and 3+19.
A valuable guide to implementing hullsplit strategic deficit irrigation and more information on pressure chambers is the publication “Drought Tip: Drought Management for California Almonds.”
Another good reference is the article “Irrigation Rates and Hull Rot” at TheAlmondDoctor.com.
You will find information on leaf tissue analysis and nitrogen budgeting at Almonds.com/Nutrients under Nitrogen Budgeting.