Perennial Canker of Almond

Written by Brent Holtz, UCCE San Joaquin County

Symptoms of perennial canker
of almond. Photo by
F. Niederholzer.



Perennial Phytophthora Canker has killed many almond trees. Two plant pathogenic fungi, Phytophthora cactorum and Phytophthora citricola are primarily responsible. This disease is lethal, perennial, and is not associated with pruning wounds which differentiates it from “Pruning Wound Canker.” Perennial Phytophthora Canker kills almond trees by girdling the scion and is usually initiated in a conducive environment either below the soil surface or on the tree trunk where the trunk and branches join.

Phytophthora cactorum is usually associated with infections initiated near the soil surface, while Phytophthora citricola typically causes aboveground infections initiated near tree branch crotch pockets. Dr. Greg Browne, USDA-ARS, has found that the source of inoculum for above ground infections is likely debris from the orchard floor containing fungal spores that are blown onto trees during harvest. This debris and spores are most likely washed off the tree during rains, with some accumulating in main-branch crotch pockets or depressions where main branch scaffolds join the tree trunk. These scaffold pockets containing soil, water, and fungal spores most likely offer a conducive environment for Phytophthora infections to take place.

Dr. Browne’s research has provided almond growers with some control measures that can be taken to prevent Perennial Phytophthora Canker. He has shown that phosphonates (e.g. inorganic and organic salts of phosphonic acid) have provided systemic activity against a number of diseases caused by many of the Phytophthora species of plant parasitic fungi. The phosphonic acid derived from phosphonates probably disrupts Phytophthora growth and appears to intensify almond tree defenses against infection. Fortunately, the phosphonate can be translocated both upwards and downwards in the tree due to its mobility in both xylem (water going up) and phloem (sugars going down) tissues.

In trials performed by Dr. Browne, a preventative foliar spray with phosphonate in the fall or spring of roughly 2 pounds per acre active ingredient (2.2 kg ai/ha) suppressed the development of Phytophthora cankers after wound inoculations of the fungus were made 15 days to 5 months after spraying. The Phytophthora cankers on trees sprayed with phosphonate before inoculation were 22 to 98 % smaller than those on trees that received no phosphonate. Dormant foliar applications of phosphonate were ineffective, probably because the dormant trees lacked the expanded l eaves necessary to absorb phosphonate. Dr. Browne also found that preventative chemi gation of phosphonate into irrigation water in the spring or summer also inhibited Phytophthora canker expansion. Chemigation was only effective during periods of high crop evapotranspiration and not during periods of low water use.

The sources of phosphonate used in Dr.Browne’s experiments included Phostrol (Nufarm Americas, Inc.) at 0.41 gallons/acre (3.8 liters/ha) and NutriPhite P foliar (4-30-8) applied at 0.46 gallons/acre (4.3 liters/ha). NutriPhite P Soil (0-60-0) was applied via chemigation at 0.46 gallons/acre ( 3.9 pounds phosphonic acid/acre).

In orchards where Phytophthora has been a perennial problem, I strongly suggest that you apply preventative phosphonate sprays in either the fall or spring.

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5 thoughts on “Perennial Canker of Almond

  1. Hi Brent,
    Do you know what the optimal concentration of phosphonate in almond roots is to prevent or minimise phytophthora issues?

    Regards
    Leigh


    1. Leigh – good question. Studies regarding the concentration of phosphonate within plant tissues for Phytophthora control have been done on several plants – mostly in work out of Australia. A study by Smillie and colleagues (1989) has shown that concentrations of 80-800 micrograms/milliliter of phosphonate inhibited Phytophthora growth in media. These concentrations were found within plants, and thought to directly inhibit pathogen growth. Other studies have shown that direct inhibition of the pathogen is not necessarily needed in order to reduce disease incidence. Grant and colleagues (1990) shown that low levels of phosphonate alters Phytophthora metabolism without affecting fungal colony growth. With this data, they concluded that the fungus in this altered state becomes more recognizable to the plant, triggering the plant defense responses, making the plant less susceptible to infection even though direct pathogen inhibition by the fungicide does not occur.

      So what does this all mean? On the field level it is important to make an application of phosphonate that will either indirectly or directly affect the Phytophthora. We generally suggest/advise a rate of 1-2 quarts/acre (2.5-5 L/ha) of a phosphorous/phosphonate/phosphite product that contains at least 40% active ingredient. In highly infested orchards, the higher rate should be used. Products with less active ingredient need to be applied at a higher rate in order to achieve the same concentration of phosphonate within the plant tissues. This should be applied foliarly in the fall (postharvest period), and in severe cases in the spring. Soil chemigation has been shown to be less effective than foliar sprays.

      We do not know the concentration that is needed to inhibit fungal growth in almond roots- we just know that these rates of phosphonates are effective in reducing disease.

      I hope this helps.


  2. Thanks David. I read some Queensland DPI research in avocados that recommended 25-40 ppm in the roots as being a minimal level for disease suppression and thought there might have been some similar work done looking at almond roots. We were thinking of doing some testing ourselves this coming season in Australia. Typically in Australia they have applied phos acid to the soil post-harvest (5 L/ha of a 60% concentrate). We are looking at post-harvest & early spring foliar sprays this season.

    D you think there be any benefit in a mid-season foliar spray in problematic or wet areas?

    Regards
    Leigh


    1. I should have known that the Australians have experience with the root concentrations with phosphonates! Most of the work with these products has been out of Australia. The work that I am most familiar with is within walnut, almond. In walnut, canker growth was reduced with chemigations and foliar sprays, as well as an additive effect of combining chemigations and foliar sprays (Browne, et al, 1996). Some work in almond by Browne and Viveros has shown an effect of chemigation and foliar sprays, but the biggest bang for the buck was a foliar spray.

      In regards to multiple in-season applications – I think there may be merit. It all depends on the persistence of the product within the tree roots. My best guess is that it slowly “leaks” out and is converted to phosphate. In that regard some type of half life of the product within the root system probably can be described, thus providing a rough estimate of when the next application should be applied. Perhaps once every three-four months?

      We have seen benefits from in-season sprays when dealing with perennial cankers in a relatively short period of time. We go from actively isolating the Phytophthora before the spray to not being able to isolate two days after the application. With highly infected walnuts (crown/root rot), we have found that a two spray program (spring/fall) to be very effective from a disease management standpoint.

      What have you all tried down your way? I know there is widespread presence of P. cinnamomi, which is relatively limited in CA. What has been the most effective treatment for managing Phytophthora? Have repeated applications helped reduce disease incidence?


  3. Hi David,
    Typcally the industry applied phosphonate to the soil post harvest at 4-5 L/ha. With the breaking of the drought and wetter seasons in the last 2-3 years many growers have also added an early spring application, again to the soil. I don’t have contact with all growers in the industry, but I would say in-season foliar sprays have not been widely used in the past and were not APVMA approved. The latest APVMA permit allows for 2 foliar sprays per season at 200g ai/100L water.
    I would say the soil application has done a reasonable job in preventing tree death however we are seeing a more non-lethal phyto infections in many orchards and this is obviously having some impact on growth and yields. The hope is that the higher efficacy of foliar sprays will address the persistent phyto issues but it may be 1-2 seasons before we get a definitive answer. The return to drier climatic conditions will also have a major impact no doubt.

    Regards
    Leigh


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