As almonds reach kernel fill and hullsplit approaches, growers need to think about hull rot. This disease is caused by two pathogens that are commonly present in the orchard and thrive on split hulls of well-fed and -watered trees.
The best defense against these pathogens, therefore, is to cut back on irrigations and to confirm nitrogen rates meet - and don’t exceed - tree and crop demand.
The highest incidence of hull rot occurs on the Nonpareils variety with Butte and Winters the second and third, respectively, in order of susceptibility.
The two primary pathogens, Rhizopus stolonifer and Monilinia fructicola, infect fruit and can cause spur and limb dieback, potentially reducing future crops.
Strategic deficit irrigation
Employing strategic deficit irrigation (SDI) entering hullsplit until 90 percent hullsplit, a period of about two weeks, can reduce hull rot by 60 to 90 percent.
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.
After two weeks of SDI, it is important to resume full irrigation until the harvest dry-down period.
It should be noted that research conducted to find the optimum tree stress level for hull rot management also found that the reduced irrigation did not affect yield or kernel size. In fact, it appeared to even out hullsplit and harvest.
Many growers initially reduce applied water by 50 percent 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.
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 accompanied by soil moisture monitoring to ensure deep moisture is not depleted, which would impose severe stress that is difficult for trees to recover.
The role of nitrogen
Long-term studies performed by University of California researchers found a strong relationship between nitrogen (N) rates and hull rot incidence — the more N applied then the higher the incidence of hull rot.
To avoid excess N, a leaf tissue analysis of nitrogen content should be conducted to determine N status. Nitrogen rates should be adjusted according to crop load as specified in nitrogen budgeting protocols outlined at the Almond Board website (see below).
Research suggests that summer nitrogen applications increase hull rot incidence; nitrogen should not be applied after kernel development is completed, typically in late spring, though it may extend into early summer. N applied after kernel development goes to the hull, increasing the chances of infection.
When it comes to treatments to help manage hull rot in high-pressure situations, the two pathogens - R. stolonifer and M. fructicola - require different management strategies. As these pathogens grow, they produce toxins. For instance, as R. stolonifera 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 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.
Even with irrigation management, nitrogen management and chemical controls available to manage hull rot, growers must still take into account local conditions, including the potential for a late rain, a late frost that could reduce crop load, and heavy-versus-coarser soils.
But even with this variation in environmental conditions, hull rot can be successfully managed by carefully monitoring tree water status through stem water-potential readings, leaf tissue analysis for N status, and monitoring hullsplit timing for optimal treatments.
A valuable guide to implementing hullsplit SDI and more information on pressure chambers is the publication “Drought Tip: Drought Management for California Almonds,” a free, downloadable PDF at http://anrcatalog.ucanr.edu/pdf/8515.pdf.
Another good reference is the article “Irrigation Rates and Hull Rot” at www.TheAlmondDoctor.com.
For information on leaf tissue analysis and nitrogen budgeting, please go to www.Almonds.com/Nutrients.