Nitrogen increases hull rot and interferes with the hull split phenology in almond (Prunus dulcis)

• Nitrogen application increased the incidence of hull rot in fruiting spurs.
• Inoculum and irrigation caused increased infection between years.
• Nitrogen increased hull rot susceptibility, but not residence time in each stage.
• Hull rot infection in high yielding trees promotes alternate bearing.

The effect of nitrogen (N) nutrition on the occurrence of hull rot (caused by Rhizopus stolonifer) in almonds was measured in a heavily infected commercial orchard in 2011 and 2012. Starting in 2008, trees were grown under differing N rates (140, 224, 392 kg/ha). In 2011 spurs in these trees were tagged by type (non-fruiting, one-fruiting, two-fruiting) and evaluated for hull rot incidence after harvest. It is demonstrated here that N application in excess of tree demand significantly increased the incidence of hull rot in fruiting spurs in both 2011 and 2012. In 2012, hull split rate was also observationally recorded every other day in the N treatments of 140 and 392 kg/ha. N application rate altered the pattern of fruit development and delayed the transition from the ‘closed hulls’ stage to the ‘beginning of hull split’ stage. However, this delay was not critical in the increased incidence of hull rot and hull infection rates did not significantly differ as a function of difference in time spent within different hull split stages. In conclusion, excess N resulted in higher hull rot incidence due to a higher susceptibility of the hulls, but not due to a longer residence time within a particular stage once the fruit started to split. Possible causal mechanisms of N effect on hull rot and the significance of the disease are considered.