Regulated Deficit Irrigation in Almonds to Manage Hull Rot

Phoebe Gordon, UCCE Advisor

Hull rot is the common name for a disease that can be caused by three pathogens: Rhizopus stolonifer, Monilinia spp., and more rarely Aspergillus niger.  The disease starts when hulls begin to split, which creates a natural wound and allows invasion of the pathogen.  As hullsplit progresses, the pathogens will spread through the hull and begin to produce a toxin, fumaric acid, that moves through the phloem and into the spur and shoot, killing the plant tissue.  The damage can be extremely severe, particularly in the lower canopy as those branches will not be replaced. 

Image 1: Photo by Phoebe Gordon. Almond shoot strike on a tree severely affected by hull rot. Note the shriveled leaves on the branch, and the carpet of dead leaves on the orchard floor.

While there are effective fungicides that can be sprayed to reduce the severity of hull rot (a correctly timed fungicide spray can reduce disease severity by up to 60-70%), the disease is directly related to high tree nitrogen fertilization and water status (it’s nicknamed the good growers disease), and managing these is the best way to reduce disease severity.  Chemical treatments should not be done in the absence of cultural practices.  You can go to ipm.ucanr.edu to look at products that are effective against hull rot as well as find the almond fungicide efficacy tables. 

Trees that are well irrigated and fertilized with nitrogen are more susceptible to hull rot.  This is because the plant tissue itself is more vulnerable to infection.  Well fertilized and irrigated trees also progress through hull split more slowly, and while it has been commonly believed that the severity of disease became more pronounced with the length of time nuts were in hullsplit, work down by Saa et al. (2016) showed that this is not the case when it comes to nitrogen fertilization.  While early hullsplit progression was delayed in trees fertilized with higher rates of nitrogen, the length of time that fruits were in hullsplit did not increase disease severity. 

Image 2: Photo by Phoebe Gordon. Almond hull with Rhizopus hull rot (right).

The ways to culturally reduce hull rot are to fertilize based on your crop load, to cut off pre-harvest nitrogen applications after kernel development has been completed (which ends roughly between the end of May and early June), and to use deficit irrigation at hull split to induce moderate water stress.  How should you manage deficit irrigation at hullsplit?  First, it is best to do so with the use of a pressure chamber.  The pressure chamber is the only way to correctly assess plant stress; ET is only an estimation of how much your trees have lost, and soil moisture sensors are only as good as how well they represent your entire orchard; if you have several soil types with very different water holding capacities and only one soil moisture sensor, you do not have an accurate picture of your orchard’s soil water status, and you also have no idea how stressed your trees are, unless they are so stressed they are wilting.  I’ve heard performing reduced deficit irrigation without a pressure chamber as playing Russian Roulette: you could get lucky and get it right, but things could also go very, very wrong. 

The best way to manage RDI is to induce moderate stress in almonds at the initiation of hullsplit.   Reduce irrigations by 10 to 20% in the first two weeks of hullsplit.  Monitor stress with a pressure chamber – you should aim to keep the trees between -14 and -16 bars.  I’ve talked to a grower who let their trees get a little too stressed during this period and suffered a lot of sticktights, so again, induce RDI without using the pressure chamber at your own risk!  Severe water stress has also been found to delay hull split (Goldhamer et al., 2006) so it is very possible to take things too far.  It should be noted that Goldhamer did not find an increase in sticktights with deficit irrigation in that study.

Pre-harvest deficit irrigation can have a slightly negative effect on yield.  Tievodale et al. (2001) performed research on deficit irrigation in advance of harvest and found that short and severe pre-harvest deficit irrigation reduced leaf and shoot death (which again is the real problem with hull rot).  In this study, kernel weight was reduced with the imposition of 50% ET during hull split, though the reductions in kernel size were less severe than the protection of fruiting wood.  Goldhamer et al. (2006) also found that preharvest deficit irrigation reduced yield via smaller nuts, but found that it did not reduce next year’s fruit load (i.e. the number of nuts on a tree).  It has been shown that postharvest water stress DOES decrease next year’s fruit load, so make sure you can resume full irrigation after you’ve done your two weeks of deficit irrigation at the beginning of hullsplit.  If you have an orchard that is severely affected by hull rot, the small yield reduction via smaller kernels may be far outweighed by the avoidance of the loss of fruiting wood. 

Cited studies:

Goldhamer, David A., Mario Viveros, and Mario Salinas. "Regulated deficit irrigation in almonds: effects of variations in applied water and stress timing on yield and yield components." Irrigation Science 24.2 (2006): 101-114.

Saa, Sebastian, et al. "Nitrogen increases hull rot and interferes with the hull split phenology in almond (Prunus dulcis)." Scientia horticulturae 199 (2016): 41-48.

Teviotdale, B. L., D. A. Goldhamer, and M. Viveros. "Effects of deficit irrigation on hull rot disease of almond trees caused by Monilinia fructicola and Rhizopus stolonifer." Plant disease85.4 (2001): 399-403.