Phytophthora, Root Rot and How to Recover
Updated: Jan 30, 2020
Phytophthora can torment farmers throughout the growing season and cause severe economic loss. Trees and vines that are only a few years old just starting to come into peak production lost without any signs of insect damage or any visible disease. The once healthy plant seems to fade away. Once symptoms of phytophthora are noticeable, the plant has already been battling against it for some time, and it affects trees, woody plants, and vegetables alike.
Phytophthora comes from the Greek words phytón and phthorá, meaning "the plant-destroyer."
But I have heard many farmers call it by names that should not be uttered in public. Different species of phytophthora have been responsible for some significant problems throughout history, from the Great Irish Famine in the 1850s to the current problem with Sudden Oak Death in Northern California and Oregon.
For phytophthora to prevail in an agricultural setting, three things are needed; water warmth and room to grow. Phytophthora is a genus of plant damaging Oomycetes, commonly known as water molds, often mistaken as a fungus. It is not anaerobic in nature but is extremely close, and control begins by introducing oxygen or carbon dioxide into its environment. The Phytophthora species that plague agricultural fields need warmth. Phytophthora reproduces at rates that are most problematic for plant health at soil temperatures ranging from 25oC to 29oC (77-86oF). The growth rate is nearly linear from 10oC-29oC and begins to decrease sharply after that (1). The final thing need for phytophthora to proliferate is room to grow freely. Phytophthora does not compete well with other bacteria and fungi in the microbiota, limited by a healthy soil's biodiversity.
There are two effective ways that growers can battle flair ups of phytophthora; phosphite fungicides and through biological controls. There are other methods of control that are used and work in some settings but have little effect on others. There are also effective methods that are very costly. Digging up every plant, letting the roots dry to kill the spores, and replanting in clean dirt is not economically feasible in an agricultural setting. The mode of action of phosphite fungicide is to weaken the cell walls until they become weak enough that they disintegrate or burst from the pressure coming from within the cell. Phosphite fungicides can have an effect on the soil and to the area of the plant that it is directly applied. The main drawback of a chemical management strategy is that the target plants often remain infected but nonsymptomatic for prolonged periods and thus go untreated. When a system like this occurs, small environmental shifts can produce uncontrollable epidemics that require multiple applications to treat the symptoms, yet they do not fix the problem.
The other way to help control outbreaks of phytophthora is with biological activity, Plant Growth Promoting Bacteria (PGPB). PGPB live in the same sphere as phytophthora; as mentioned, phytophthora does not compete well. There are several different species of soil microbes that affect phytophthora; the modes of action vary from adding CO2 to the environment, which stresses the pathogen, to direct antagonism. Some strains of biology can even help the plant build a resistance to the disease, stimulating the plants own defense systems. Boosting a plant's systemic acquired resistance (SAR) and Induced systemic resistance (ISR) can only be done through specific biological strains. Certain PGPB have antibiotic-producing capabilities enabling the plant to not only fight off the pathogen affect but recover infected areas. By having good biological activity in the soil you are not only treating the symptoms of phytophthora, you are also working to fix the problem.
Cultural controls and preventative measures are more productive and more economically efficient than trying to regain control after infections have started. The most crucial cultural practice is appropriate irrigation. Irrigation management is critically important in fields that are already diseased as well as in the margins of infected areas. Many irrigation management options can help. Please take advantage of local evapotranspiration rates to help schedule the frequency and amount of water needed. Subsurface moisture monitoring like tensiometers are useful tools or even replacing sprinklers in affected areas with lower output emitters. Regions that are saturated or have standing water should never have more water added. Do not settle for you current situation there are always option for success.
Increasing the correct microbial activity has a myriad of benefits for controlling phytophthora: improving soil structure, increasing the C>N ratio, reduced soil compaction, and stronger root growth are all benefits of having a healthy microbiota. Proper balanced nutrition is vital: excessive fertilization of nitrogen, copious amount of animal manures, or over-application of fertilizers that are high in salts can weaken the plant and promote the growth of phytophthora or other pathogens. When replanting, use rootstocks that are resistant or more tolerant of root rots and certified disease-free to provide the best chance for replanting success. Remember, there is no silver bullet to any issue; it is always a combination of best practices that yield the best result.