Root and crown rot of fruit and nut trees have caused major losses within orchards around the world. First reported in California in 1912, Phytophthora, which means “Plant Destroyer” in latin, causes root and crown rot throughout the San Joaquin and Sacramento valley. The problem tends to be more severe in areas of clay to clay-loam series soils, which can be attributed to the higher water holding capacity and poor drainage of these soils in comparison to sandier soils.
The effect of Phytophthora spp. on root systems is often not noticed until the above ground parts of the tree show symptoms. Symptoms of Phytophthora damage are usually observed by poor tree vigor, loss of scaffold branches, and gumming or bleeding found around the tree trunk. Orchards affected by Phytophthora crown rot can suffer tree loss quickly, especially during the spring when winter-infected trees collapse during the first hot spell and during periods of high heat in which over-irrigation easily occurs. This is in contrast to tree loss that is attributed to root rot. In this case, by the time most growers recognize the symptoms, several trees are in a declining state due to a long period of infection (several years). Careful observance of the orchard can reveal problems and warrant remediation before the manifestation of severe orchard losses.
Figure 1: Overall view of a walnut tree with Phytophthora. Note the collapse of the tree from top to bottom. As the disease infects the roots or crown, flow of water to the highest branches is compromised, with upper branch dieback being the first above ground symptom.
In the orchard, symptoms of crown rot usually include cankers that are centered at the crown or lower part of the trunk. Removal of bark in affected areas will reveal brown or black sap-wood that may be gum soaked with stone fruit trees or slimy with pome and walnut trees. Cankers are clearly delineated, with very little dis-colorization in the wood that is found outside the necrotic areas. Removal of soil from around the trunk will reveal the canker’s infectious path, moving up the tree from the soil. In some cases, cankers may extend some distance up the trunk. Crown rot kills the tree through phloem damage which prevents the conduction of nutrients up the tree.Figure 2: Above ground symptoms of an almond tree with Phytophthora root rot. Note the pale, sparse foliage found throughout the canopy of the tree.
In the case of Phytophthora root rot, cankers found above the soil line are rare. In this case, the disease severely affects roots of all sizes, removing the trees ability to pull water and nutrients from the soil. Trees suffering from root rot will have black, mushy roots which can be observed upon excavation. Roots and soil may have a “rotten egg” smell. This smell, however, is not from the disease itself, but from the anaerobic conditions caused by excessive soil moisture. Tree death occurs when a substantial part of the root system is infected, causing tree collapse due to the inability to pull water and nutrients from the soil.Figure 3: Bark removal shows Phytophthora canker on walnut crown. A defined line marks the margin of infected and healthy tissue.
Isolations from California fruit and nut orchards have revealed over 16 different species of Phytophthora infecting and causing tree decline. The disease agent is typically introduced into orchards through plant material, infested soil, or contaminated irrigation water. Once a susceptible host becomes available and is infected, rapid population growth occurs, producing survival structures that are able to survive for several years, if not decades, within the soil. This process provides an opportunity for Phytophthora to develop resistance towards chemical control measures such as mefenoxum (Ridomil) and fumigation.Figure 4: An almond tree showing symptoms of Phytophthora crown rot. Bark removal shows the margin of the canker crossing the graft union and moving into susceptible scion. Scions of almond trees tend to be highly susceptible, with quick movement of the disease. Disease progresses much slower on the rootstock.
Field observations support the conclusion that wet and cold climates will increase Phytophthora loss in an orchard. This is due to these weather conditions being conducive to pathogen growth. Soils that are constantly wetted and kept at or above the soil saturation point have higher disease rates than trees watered slightly below saturation point. This can happen through winter rains or improper irrigation management. Periods of rainfall or water standing for over 24 hours can provide enough moisture for Phytophthora infection, regardless of the season. Cooler temperatures do not inhibit the growth of Phytophthora, and may increase pathogen reproduction thus increasing disease.
The first step in disease management is prevention. Disease spread can be reduced by cleaning equipment that may have been used in areas infested with Phytophthora. When planting a new orchard, choose locations that do not hold water. Plant the trees high on raised berms, being careful not to have the soil line above the graft union as the scions tend to be highly susceptible to Phytophthora. Use of resistant rootstocks is strongly encouraged.
Since water plays a key role in disease development, irrigation practices should be modified in areas at risk for Phytophthora to more frequent, shorter watering periods. Over-watering should be avoided, especially during periods of low water use by the tree. Also, irrigation sets should not exceed 24 hours. Furthermore, use water guards to prevent water from splashing on trees, employ practices that promote good water infiltration and penetration, and try to reduce soil compaction.
Even though proper water management is critical in disease prevention for healthy trees, it has a minimal effect on trees that are currently infected. Research has demonstrated the effectiveness of fall foliar applications of Phosphorous acid (NOT PHOSPHORIC ACID!). Apply 1-2 applications to trees that are fully leafed out on 2-4 week intervals. Soil applications and dormant sprays of phosphorous acid have provided some disease suppression, but use higher rates and are not as effective as foliar sprays. Phosphorous acid has only been demonstrated to control Phytophthora diseases; therefore, applications suggested for other purposes have not been proven by research and may be unnecessary.
Use of genetic resistant or tolerant rootstocks to help reduce the impacts of the disease has been an area of focus in many research trials. Within walnut, Paradox rootstock is significantly more resistant to P. cactorum, P. citrophthora, P. drechsleri, and P. megasperma, and is somewhat more resistant to P. citricola and P. cinnamomi than Northern California Black rootstock or English rootstock. Furthermore, newly developed Paradox genetic lines have demonstrated increased resistance to P. cinnamomi and P. citricola in comparison to the currently available Paradox rootstocks. In greenhouse studies, RX1 has exhibited resistance to both of these Phytophthora species, while VX211 has expressed resistance to P. citricola. These rootstocks, however, have not been thoroughly field tested. Within almond and peach, in comparison to Nemaguard and Lovell, Peach/Almond Hybrid rootstocks are more susceptible to Phytophthora, while Marianna 2624 is less susceptible towards the disease. Viking and Atlas have the same level of resistance as Nemaguard and Lovell.
Since not all Phytophthora species are the same, the performance of resistant rootstocks will vary by which species is present. Therefore, it is necessary to confirm which species of Phytophthora is present before orchard replanting. Contact your local farm advisor for sampling of suspected diseased trees before removal. For all tree crops, rootstocks provide a line of defense, but are only one piece of the puzzle. It is best to apply as many, if not all, orchard management practices in order to successfully manage Phytophthora.