I have been getting a few calls regarding hull rot. I have covered this topic before (link to previous article), but some of the particular questions regarding the disease are answered below.
What is Hull Rot?
Hull rot is infection of the almond hull by two fungi, Rhizopus stolonifer (common bread mold), or Monilinia fructicola. These pathogens are common throughout the environment, and are, in this case, serving as opportunistic pathogens. Upon infection, they release toxins that are translocated into the fruiting wood, which kills the wood, causing crop loss.
What do you mean by opportunistic pathogen?
Once the hull splits, the perfect micro-climate for fungi is created. The hull is full of nutrients and water – the two things that fungi want from a host. Since the spores of these fungi are found throughout the air, they invade the newly split tissue, infecting, and completing their life cycle. By reducing the “home” for the fungi, we can reduce the number of hull rot strikes. These strategies include reducing the water and nutrient content of the hull.
How can I prevent Hull Rot?
Asking “How to prevent” is a good start. Prevention is the key to reducing hull rot strikes. Hull rot often affects high vigor orchards. Nitrogen should not be applied after kernel development is completed. This is typically the end of May, but this year it extended into mid-June. A slight to moderate water stress at the onset of hull-split should be applied. Applying both of these practices to the orchard has been shown in multiple research trials to reduce hull rot by 80-90%. Often times, this is all growers need to do to prevent severe hull rot issues.
I tried those things, I still have hull rot!
Severe outbreaks of hull rot have been observed. Last year, hull rot infections occurred in many orchards. I suspect that this year will be the same. A drawn out, late hull-split will make it difficult for growers to achieve the proper dry-down for the hulls. Shorter days means more humidity and less drying time. Research studies have shown that fungicides applied at 25% or less of hull-split will reduce the incidence of hull rot. These studies were performed by Dr. Jim Adascaveg last year (2010), in which he shown effectiveness of most fungicides, especially the strobilurins. I have checked a few labels, and have not been able to determine if they are registered for this use, so growers need to check with their PCAs. If fungicides are used, they should be used in combination with cultural practices. Maximum residue levels (MRLs) should be discussed with the processor/handler to determine the most up-to-date information, and pre-harvest intervals should be followed.