I have been in several orchards this year which have been determined to have Phytophthora Crown Rot.

This disease is aggressive and can cause the rapid decline of any aged tree. Conditions that favor disease include excessive periods of saturated soils and cooler temperatures common in the late winter and early spring.

An infected tree can be identified by a rapid collapse of the canopy and the observation of a canker extending from the soil. Often, when soil from around the crown is removed, the canker can be observed.

The occurrence of Phytophthora has become more common over the past few years. Reviewing my records over the past seven years, farm visits regarding Phytophthora disease diagnosis and management averaged about three per year.

Over the past two years, I have seen a 133 percent increase in annual visits. My notes suggest that this is due to lower quality soils where orchards are planted, the poor selection of rootstocks for these soils, and mismatched irrigation scheduling for the soil type and tree size.

In managing this problem, the industry has relied on several cultural and chemical control methods. Below are a few of the cultural practices, chemical management, and problems/concerns regarding Phytophthora root and crown rot.

More information can be found at the UC IPM website.

Rootstock selection. More peach-almond hybrid rootstocks are being planted in new orchards. These rootstocks are more vigorous and tolerant of toxic salts, but are susceptible to Phytophthora. In general, plum parentage rootstocks (e.g. Marianna-2624) are the most tolerant.

Peach rootstocks, which include Nemguard and Lovell, are moderately tolerant, while other complex hybrids (Viking and Atlas) have demonstrated some tolerance in greenhouse tests. Peach-almond hybrids (e.g. Hansen 536) are the most susceptible. Newer rootstocks remain untested (e.g. Krymsk-86, Empyrean-1, etc).

In some cases, varieties are being planted on different rootstocks (e.g. ‘Nonpareil’ on Nemaguard, “Wood Colony” on Hansen 536) in order to increase vigor for the weaker growing variety. This has created issues in orchard establishment due to mismanaged irrigation during orchard establishment. Too wet soils will cause increased losses on more susceptible rootstocks.

Common causes

Cultural practices. Planting the tree properly and planting the tree on a mound or berm helps reduce infection. The tree should be planted high – no deeper than the nursery line- and the graft union should be above the soil as the almond scion is highly susceptible to infection. The mound or berm should be pulled prior to planting and shaped to allow water to drain away from the crown.

In soils with low infiltration rates or high holding capacities, the duration of irrigation should not exceed 24 hours, and preferably, standing water should not be present. If so, soil infiltration rates should be improved or shorter irrigation sets should be applied.

Saturated soil at the base of the crown should be minimized. Sprinklers and drip irrigation lines should be moved as the tree ages and water applications increase to prevent infection.

Over-irrigation in the spring. This is probably one of the more common causes of Phytophthora in heavy soils. Pulling the trigger too soon on the first fertigation event creates water logging – especially when spring rains may occur. Furthermore, saturated conditions delay the development of the trees, often giving them a yellowish color, suppressing leaf development and transpiration.

Subsequent irrigation will begin to kill the roots due to anaerobic conditions, and Phytophthora will become established.

Marginal soils. A fair share of my visits has yielded orchards planted on soils with high salts and heavy soils. Rootstocks selected to manage one of the problems often lead to susceptibility to the other. Salinity management often requires leaching which can create saturated soils.

I have often advised to select a rootstock to address the long term problem, and implement the best cultural practices for the other, but this may not always be possible.

If challenging soils are planted, tree and subsequent yield losses should be expected.

Water quality

Poor water infiltration. This is becoming more of an issue with a reliance on lower quality of water. Soils will “seal” as different minerals are applied through the irrigation water. Maintaining a sodium absorption ration (SAR) under three helps maintain a soil chemistry that is conducive for higher infiltration rates. Gypsum or lime is often used to add calcium for this purpose.

There is some evidence within other cropping systems that cover crops, organic matter, and humic acids may increase infiltration rates. University research has shown benefits of cover crops, but University research within almonds for the other two products is limited.

Soil compaction should be minimized as this impacts infiltration rates.

Cover crops can also help. It may also help to allow ample room to turn around at the end of the orchard as too close of a pass to the first tree will compact the soil around its roots.

Vertebrate Pest Damage. Gopher, vole, and field mice can damage the crown of the tree, reducing the rate of water uptake. If the trees are irrigated at the same schedule as unaffected trees, they will be over-irrigated leading to infection. Vertebrate pests should be controlled with baiting or trapping.

Chemical management. For the most part, the industry has relied on the use of Phosphonates (also known as phosphites and phosphorous acid products) applied as a fungicide or foliar nutrient. These products are highly effective in suppressing this disease. Apply one to two applications to trees that are fully leafed out on a two- to four-week interval.

Repeated use through the season has not shown any added benefit, but a third spray made about six months later in the fall prior to leaf drop may help reduce infections during the winter.

Recently, however, a trade issue has been determined between the European Union and the United States regarding the maximum residual level of Phosphorous Acid detected within the kernel. This issue will take several years to be resolved, and until then, the Almond Board of California has requested that the products not be used. 

If a phosphonate product needs to be sprayed, it is important to contact your processor to determine if it is possible. It may be possible to redirect nuts away from the EU market.

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Other chemical control methods include the use of mefenoxam (Ridomil Gold). This product has also been shown to be effective in reducing soil populations of Phytophthora. Applications should be made in the early spring or late fall to reduce populations prior to periods of favorable environmental conditions that occur during these times.

Editorial note: Dr. David Doll is a University of California Cooperative Extension farm advisor in Merced County. His expertise includes almonds and other tree nut crops. This article originally appeared in The Almond Doctor Blog and is reprinted with Dr. Doll's permission.