Almond Hull Rot: Management Strategies to Lessen Application Needs

One of the prettiest sights to see in the California springtime is fully bloomed almond trees covering farmland up and down the valley. But what about the orchards lacking bloom? Well, hull rot may be the culprit.  

Hull rot can limit the full potential of an orchard, hindering bloom the following year. However, there is good news. By implementing effective cultural management techniques, such as proper water and irrigation practices, you can proactively minimize the chances of hull rot occurring in your orchard. 

Identifying Hull Rot 

Like any disease, hull rot occurs when an aggressive pathogen and susceptible host are present in an ideal environment. However, hull rot isn’t like any other disease.  

Hull rot is known to attack well-maintained, heavy-cropped orchards, especially in those with more susceptible varieties such as Nonpareil, Butte, and Winters. Perhaps the most threatening trait of hull rot is its effects on future harvest. Serious disease can kill the tree’s spurs, which produce the bloom for the following year. Once these spurs are lost, next year’s harvest could be in jeopardy.  

There can also be a loss in fruiting wood, especially in the lower parts of the tree, which is why many growers mistake hull rot for shading out, said Brent Holtz, Ph.D., county director and farm advisor at University of California Cooperative Extension (UCCE). All these factors have a large impact on yield and on the productivity of the orchard.  

Given the correct signs, hull rot can be fairly easy to detect. Mohammad Yaghmour, Ph.D., orchard systems advisor at University of California Cooperative Extension (UCCE) noted that it’s important to know the difference between disease signs and symptoms. While symptoms are the tree’s reaction to the disease, the signs will tell the grower what kind of pathogen, or causal agent, they are treating. 

“The most important part before we even think about the management of any disease is to have the correct diagnosis, and to have the correct diagnosis, I tell the growers to look for the signs,” Yaghmour said.  

Hull rot always occurs during or at the end of hull split, so growers know it has infected a tree when leaves appear dry or shriveled up during the first few weeks of July. Clusters of dead leaves are easily seen among the healthy foliage. Once the hulls are split, further evidence shows the damage the fungus has caused to the spur.

Causal Agents 

There are three major pathogens that cause hull rot. More prevalent in the northern part of the valley is Monilinia fructicola, an airborne spore which causes a brown area and tan fungal spores on the outside of the hull. Aspergillus niger is another causal agent primarily in the San Joaquin Valley, where flat, jet-black, powdery spores will develop between the hull and shell. Similarly, Rhizopus stolinifer is the third common causal agent also in the San Joaquin Valley. Spores grow between the hull and the shell and are fuzzy and black/gray in color.  

Soil is a major source of Aspergillus niger and Rhizopus stolinifer, and any movement or wind can be especially disastrous when dealing with these causal agents. That’s because the creation of dust may carry spores into the air, putting the split hulls at risk for infection. Rhizopus stolinifer infection will result in production of fumaric acid that will cause the death of shoots and fruiting spurs. An obvious symptom is the dead, shriveled leaves near infected almond fruit.  

Finding a Solution Using Cultural and Management Practices 

Certain hull rot fungi are more prevalent in some parts of California than others, but proper nitrogen and water management are universal solutions for dealing with it. 

“Orchards differ in their susceptibility,” Yaghmour said. “Whether you’re going to take a decision to manage this disease, add fungicide spray or just depend on cultural practices to manage this disease, it will depend on your own situation in your orchards.” 

Many refer to hull rot as the “good-grower disease” because research has shown that it is the consequence of too much water and overfertilization during the growing season.  

While fungicide applications are an option for managing hull rot, they are not the preferred solution. There are many diseases that can only be controlled using chemical applications, whereas hull rot prevention is better controlled with a proper nitrogen and irrigation plan. Also, the efficacy of chemical control during hull split to manage hull rot might be challenged by the high variability of hull split timing in a given orchard. 

In two trials sponsored by the ABC in 2011 and 2012 led by Sebastian Saa, Elana Peach-Fine, and Patrick Brown, it was proven that excessive nitrogen use can stimulate hull rot. The researchers applied a nitrogen treatment on various spur types, resulting in an increase of hull rot. 

A good starting point for proper management to combat hull rot is the use of the 4R’s of nutrient stewardship – right source, right rate, right time, and right place. 

As a guideline for growers, UCCE suggests that trees demand 68 pounds of nitrogen for every thousand kernel pounds of almonds produced per acre. Continuous fertigation at the right rate is also a strategy encouraged by industry experts.  

The Nitrogen Best Management Practices guide – developed by the Almond Board of California and U.C. Davis – notes that excessive tree nitrogen levels can lead to extended hull greening, prolonged vulnerability to hull rot infections and reduced shaking efficiency. The guide states that applying more nitrogen than is required in May and June, then withholding nitrogen fertilizer until after harvest, is not recommended. By doing so, a grower exposes the early excess nitrogen application to leaching loss in subsequent irrigations and may “over-feed” trees in the period immediately preceding hull split.  

Additionally, stopping nitrogen fertilizer application in May or June can deprive trees of nitrogen during critical bud formation and the period when trees accumulate nitrogen for the harvest preparation stage, which occurs from hull split to early post-harvest. To minimize the risk of these problems and hull rot attacks, growers should continuously fertilize trees at the appropriate rate and time, using numerous small applications. 

Reducing Water, Reduces Hull Rot 

To many growers’ surprise, reducing water when approaching hull split is one of the prime ways to avoid hull rot. This technique is called regulated deficit irrigation (RDI). 

“It’s not that you need to irrigate less through the whole season; it’s that you need to irrigate less at the first two weeks of hull split, so usually the first two weeks of July,” Holtz said. 

A lot of growers are hesitant to reduce irrigation during the first two weeks of July because they can be the hottest weeks of the year, and growers are fearful that it will lead to mites. While pests are no issue to ignore, Holtz emphasized that hull rot is worse than any pest because it causes permanent damage to the lower canopy wood forever.   

Further research by industry experts supports that inflicting mild stress on trees during early hull split results in significant reduction in hull rot. This research was conducted by Drs. Beth Teviotdale, David Goldhamer, and Mario Viveros, who experimented with deficit irrigation from March through November 1994 and 1995. 

The researchers irrigated trees at 70, 85, and 100% of potential evapotranspiration (ETc) using two irrigation methods: sustained and regulated. The sustained irrigation method reduced water the whole season, while the regulated irrigation method began the year at normal irrigation rates and slowly reduced water leading up to hull split.  

Irrigation frequency was the same for all treatments, and irrigation duration was always 24 hours. Deficit irrigations were applied by adjusting the sizes of the microsprinkler nozzle and operating pressures. 

Their findings showed that the regulated irrigation method was more effective at reducing hull rot than sustained irrigation. In fact, eliminating irrigation during the two weeks preceding harvest reduced hull rot by 400-500% in those two consecutive years (Goldhamer and Viveros).  

So, what can growers take away from this study? RDI is a proven technique to ensure hull rot is avoidable, and the UCCE recommends growers consult with their farm advisor to properly apply RDI and mild stress on their orchards during the first two weeks of hull split.  

Using a pressure chamber when practicing RDI to check the tree’s stress levels is also an important tool to ensure proper use of this strategy. It’s important to note that a tree’s stress levels will differ according to its unique soil profile and environmental factors.   

While managing hull rot is no small feat, extensive research shows that the time and attention dedicated to accurate nitrogen and irrigation levels are well worth the hassle to prevent this harmful disease from entering your orchard and ultimately affecting future yield. We recommend consulting with your farm advisor for further information and support to control hull rot.