Phytophthora: An Update
As previously written (2009article and 2015article), Phytophthora is a plant pathogen of almond trees. Thisfungal-like water mold is common throughout the various almond growing regionsof the world, with 6* or more known species causing disease. When conditionsare conducive for infection, the pathogen directly invades plant tissues at thecrown or roots, leading to loss of nutrient and water uptake, causing treedecline and eventual death. The rate of infection and tree loss depends on theinoculum present, the choice of rootstock, environmental conditions, andcultural practices that may favor disease.
Phytophthora inoculum can be endemic to the orchard area orintroduced through a variety of ways. Low levels of Phytophthora probably existin most soils, but typically are not at the levels or location to causeinfection unless there are prolonged conditions favoring disease. Generally, introduction of inoculum into thearea of close proximity to the plant is most likely responsible for infectionsand tree death. This includes the crown or base of the tree where the pathogeninfects the plant just below the surface of the soil. These types of infectionsare most common and can lead to rapid tree loss. Root infections, which occur when soils aresaturated with water, lead to a gradual decline. Finally, aerial Phytophthora happenswhen spores are deposited on limbs or in crotches of the trees through dust orflooding, followed by wet conditions (e.g. rain).
Surface water is believed to be the major contributor ofPhytophthora inoculum in perennial nut orchards. Surveys have found severalspecies of pathogenic Phytophthora in high elevation, upstream waterways of themajor rivers in California. Due to the ability of the pathogen to survive inwater, this pathogen is probably found in all surface water used to irrigatetrees worldwide. This inoculum is ableto move through the filtration systems due to size and is unaffected by currentaffordable and plant safe water treatments. Therefore, it is generally best toassume that Phytophthora is present in all orchards. As such, diseasemanagement should focus on rootstock selection and orchard practices thatreduce disease.
Phytophthora infection rates are strongly influenced by thepresence of water and excessive moisture. Saturated soils provide the abilityfor the spore to “swim” to the point of infection. This soil condition alsostresses the plant, making it more susceptible to infection. Even though treedecline becomes apparent in warmer temperatures, infection rates are higherduring the spring when the soil is cool, damp and the plant is active. Cautionshould be used when irrigating – do not apply water if adequate soil moistureis present.
Rootstock selection is one of the best ways to reducedisease within an orchard. Different rootstocks have varying genetic tolerancesto disease. When selecting a rootstock, the risk of Phytophthora should beconsidered as well as other horticultural characteristics. With almondrootstocks, generally, plum parentage rootstocks (e.g. RootPacR, Marianna 2624,Krymsk-86) are the most resistant to infection while almond rootstocks (e.g. seedling rootstock) are mostsusceptible. Peach rootstocks, such as Nemaguard and Lovell, generally showmoderate resistance while providing more tree vigor than plum parentage.Peach-almond hybrids (GF677, Hansen 536, Garnem and others) are highlysusceptible to infections. Finally, complex hybrids such as Viking and Atlashave resistance levels similar to peach rootstocks.
Other practices can reduce the occurrence of disease withinan orchard. Trees should be planted on a berm or mound so water can drain awayfrom the crown of the tree. Infiltration rates should be maintained through theuse of either soil amendments, reduced application rates, or larger irrigationpatterns. Drip hoses should not be placed at the base of a tree to reduce soilsaturation at the crown. Proper drainage should be installed to reducesaturated soils in low-lying areas. Finally, tree cartons or wraps should beremoved after the first year to prevent aerial infections of the trunk, as debrisand spores can collect in these areas and infect during rain events.
Although rootstock selection and cultural control practicesare ideal in reducing disease occurrence, there are also some chemical controlmethods. Mefenoxam is a fungicide that can directly kill the pathogen, reducingthe inocoulum load. This product must come into contact with the pathogen andreinfestation of the treated should be prevented. Potassium phosphite and fosetyl-aluminumapplied as a foliar spray can reduce disease severity by increasing the tree’sdefenses. These products increase the chance of compartmentalizing the disease,allowing for recovery. Soil applications of these products can also occur butgenerally do not show the same effect as a foliar treatment. Lastly, there isan increasing number of biological products that can reduce the occurrence ofdisease by either antagonism or plant protection. Due to the potential oflimited capacity of these products, for the time being, it is best to use themin combination with other chemical control methods.
Disclaimer: Mentioning ofany active ingredients is not an endorsement or recommendation. All chemicalsmust be applied following the chemical label, local and federal regulations.Please check with your pest control adviser to confirm rates and site-specificrestrictions. The author isnot liable for any damage from use or misuse.
*Phytophthora species of almond: P. cactorum, P.megasperma, and P. cinnamomi (root and crown pathogens), P. citricola (scion,aerial infections), and P. syringae and P. niederhauserii (links with nurseryinfections).
Materials referenced:
Browne, G. 2017. Resistance to Phytophthora species among rootstocks for cultivated Prunus species. Hortscience. Vol 52:11 1471-1476.
Browne G.T., Beede R.H. and Schmidt L.S. 2012.Irrigation water relation to the health of deciduous fruit and nut crops. In:C. Hong and G. Moorman (eds.). Biology detection and management of plantpathogens in irrigation water. APS Press St. Paul MN
Browne G.T. andBecherer H.E. 2000. Relativeresistance to Phytophthora spp. amongsome rootstocks for almond. Phytopathology 90:S10
Schmidt L.S., Bhat R.G., Kluepfel D.A., andBrowne G.T. 2012. Resistanceto Phytophthora innew rootstocks for almond and stone fruitsPhytopathology102S4.106abstr.)