Research funded by the Almond Board of California (ABC) over several years has shown the proper timing and amount of irrigation is of prime importance to both yield and pest management considerations later in the season. Too little moisture causing significant post-harvest stress affects fruit set in the subsequent season, but on the other hand, regulated deficit irrigation (RDI) timed to stress trees at the onset of hull split reduces the incidence of hull rot. Deficit irrigation also results in a more uniform hull split leading to earlier harvest; the latter can help minimize crop exposure to late season navel orangeworm (NOW) flights.

Hull Rot

Hull rot is the single greatest yield reducer of vigorous young almond orchards in the central San Joaquin Valley, according to Brent Holtz, UC pomology farm advisor, Madera County. He adds irrigation management is the only practical control for this fungal disease.

The severity of hull rot is related to the length of the hull split period; as long as hulls are moist, they remain susceptible to infection. Under normal conditions, this is generally two weeks after hull split on individual almonds. In orchards with vigorous growth, however, the hull split period is extended. In these orchards, the disease is more severe.

ABC-funded research by Beth Teviotdale (pathology specialist emeritus), David A. Goldhamer (irrigation management specialist), both at the Kearney Agriculture Center, Parlier, Calif. and Mario Viveros, UC pomology farm advisor emeritus, Kern County, has shown hull rot can be reduced by imposing mild water stress on trees timed to early hull split. In experiments in Kern County, hull rot incidence was reduced by half or more when trees were stressed for two weeks during early hull split.

NOW Prevention

Additional benefits of hull split stress are a more uniform maturation and this results in an earlier harvest. When nuts are harvested early and removed from the orchard promptly, a third generation of navel orangeworm egg-laying and subsequent damage on the new crop can be avoided.

Regulated Deficit Irrigation

One approach to hull rot prevention is regulated deficit irrigation imposed at the beginning of hull split. This can be monitored and quantified by using a pressure bomb to track midday stem water potentials (MSWP). Based on two years of studies in cooperation with several farm advisors, Ken Shackel, Dept. of Pomology/Plant Sciences, UC-Davis, was able to impose enough stress to reduce hull rot and yet not overstress trees to the point that made them susceptible to mite flaring, defoliation, and reduced subsequent yield. Shackel targeted -14 to -18 bars MSWP during hull split, which reduced hull rot and increased hull splitting.

A Balancing Act

Late-season irrigation management timing can be a challenge and it is best to use plant-based and soil monitoring in concert. As noted above, monitoring with a pressure bomb has proven valuable to impose short-term regulated stress for hull rot management; however, Bruce Lampinen, pomology specialist, UC Davis, cautions both soil and plant-based monitoring should be used for comprehensive irrigation management so that trees are not over-stressed. Using plant-based measurements alone can be deceptive: It is possible to have an unstressed tree as the lower soil level dries, but once the lower moisture is lost, trees go into stress very quickly. This is very difficult to manage later in the season when trees are totally dependent on irrigation water to satisfy their needs.

Furthermore, severe water stress later during bud differentiation, which continues through mid-September, has been found to dramatically reduce fruit set the following spring and should be avoided. Growers should take into consideration the following research results regarding late-season deficit irrigation and tree stress:

· In his research on low-volume irrigation systems, Goldhamer determined shallow-rooted almond trees on low-volume irrigation become stressed quickly when water is withheld. Shallow-rooted trees require post harvest irrigation if they are subjected to significant water cutoff.

· With a 25-day or less pre-harvest cutoff, there were no significant effects on yield as long as the previously well-irrigated trees received full post-harvest irrigation. In extreme situations, Goldhamer found a dramatic decrease in fruit set the next season as a result of previous post-harvest moisture deprivation. The greatest decrease (96 percent) occurred with the earliest pre-harvest cutoff, which was 53 days before harvest.

· Goldhamer concluded that in warmer almond-growing districts that have earlier harvest, growers with low-volume irrigation systems should try to irrigate just before and immediately after harvest.

· Deep-rooted trees grown under high-volume irrigation have been studied by Extension Water Management Specialist Terry Prichard, San Joaquin County. Prichard found trees grown under these conditions may have enough pre-harvest moisture to sustain them through the bud differentiation period and may not need post-harvest irrigation. This is especially true in the northern growing regions where harvest occurs later, after bud differentiation, and where fall rains are more likely to occur.