Disease symptoms caused by Phytophthora are difficult to distinguish from diseases by other microbes. Phytophthora species may infect all plant parts (fruits, flowers, leaves, stems, and roots) and cause root rots, wilting, bleeding stem cankers, shoot blights, fruit rots, and foliar lesions. In our recent survey of woody hosts in Minnesota nurseries we found many different species of Phytophthora causing severe damage. Some of the species found include Phytophthora citricola, Phytophthora citrophthora, Phytophthora cambivora, Phytophthora cactorum, Phytophthora fragariae var. rubi and Phytophthora nicotianae.
5-year old tree die due to fungal attack
The development and spread of Phytophthora diseases are favored by wet conditions. The fungus-like organism produces zoospores (motile spores) that are propelled by flagella in wet environments. Other spores, such as chlamydospores and oospores may also be produced allowing the organism to survive adverse conditions. In nurseries, if Phytophthora diseases are problematic, there are several control procedures that can reduce the impact of these diseases including chemical control, sanitation, changing cultural practices and host resistance.
Phytophthora root rot must be prevented as chemicals are often ineffective in controlling this disease. Keeping nurseries free of diseased plants is very important in disease prevention. Planting on raised beds and using porous potting media may help to reduce excessive moisture and development of Phytophthora diseases. Adding composted bark to the potting soil has also been shown to reduce disease. If recycled irrigation water is used it is essential that Phytophthora propagules are not present in the water or additional cultural or chemical control strategies may be needed. Environmental stress on woody plants also makes them more susceptible to infection by Phytophthora species. For example, drought stress, high soil salinity, and excess nitrogen fertilization may promote disease.
Below, photos showed the disease happened to my healthy trees. This Oct ~ Nov 2012 was the unusual rainy season, it rained 6 days a week. 3 trees were noticed attacked by fungal in a same location, at lower landscape. Neighbour trees showed sign of unhealthy condition also.
15-month old Sinensis, leafs fall. Attack was noticed in recent rainy season ( Oct ~Nov 2012 )
15-month old Sinensis, shoot blights. Attack was noticed in recent rainy season ( Oct ~Nov 2012 )
Managing Phytophthora Diseases
There are more than 60 described species of Phytophthora and all known species are plant pathogens. Each species can cause disease in from a few to over a 1000 different plant species. Hence, there are a few thousand diseases in a wide range of plants caused by the various species within the genus Phytophthora. Each of these diseases will have its own characteristics, which makes it difficult to generalise disease-control methods.
Microbial application such as Bacillus amyloliquefaciens is known for its plant growth promoting properties and its ability to suppress different plant pathogens.WiltRid is one of the products contain this kind of beneficial microbes.
Chemical fungicides (systemic) application
Phenylamides (acylanilides). This group of chemicals includes furalaxyl (Fongarid), metalaxyl (Ridomil) and benalaxyl (Galben). All three chemicals are active against the Peronosporales, but metalaxyl is the most widely used (Erwin and Ribeiro 1996). This fungicide is a xylem-translocated compound with an upward movement in plants in the transpiration stream (Edgington and Peterson 1977). Thus, metalaxyl and related acylanilide compounds have no effect on root diseases if applied as a foliar spray because they are not transported to the roots. Metalaxyl is usually applied as a soil drench and it is very effective (Guest et al. 1995). Due to its systemic nature, metalaxyl is transferred from seed, roots and leaves to new growth (Cohen and Coffey 1986) and is therefore effective at controlling infection beyond the roots. Metalaxyl is water soluble, and is effective against all species of Phytophthora in vitro at much lower doses than protectant fungicides. The biochemical mode of action of metalaxyl involves inhibition of RNA synthesis. It is highly inhibitory to sporangium formation, and also reduces chlamydospore and oospore formation (Cohen and Coffey 1986). It also has a high level of persistence within the plant. The presence of metalaxyl within the plant can prevent colonisation of leaf tissue by mycelium, because it inhibits the growth of hyphae (Erwin and Ribeiro 1996).
There are several disadvantages of using metalaxyl and related compounds:
- root drenching is a wasteful method of fungicide application;
- chemicals are released into soil and water systems;
- soil microorganisms rapidly degrade metalaxyl, reducing its persistence and effectiveness (Guest et al. 1995);
- resistance has developed to it among populations of Phytophthora, particularly P. infestans (Cohen and Coffey 1986). The issue of metalaxyl-resistance has been partially addressed by application of metalaxyl in combination with a protectant fungicide, limited application of metalaxyl during a given growing season, and not using the fungicide for curative or eradicative purposes (Erwin and Ribeiro 1996).
Potassium phosphite against Phytophthora
Potassium phosphite is a unique fungicide with curative and protective activity against many pathogens of agricultural, nursery, greenhouse, landscape ornamentals, turf and lawns. It is quickly absorbed by plant foliage and roots and is distributed systemically throughout the plant. Once inside it turns on the natural defense mechanisms of the plant to resist invasion by disease causing pathogens. If diseases are already present it helps the plant to kill the pathogen invaders by curative activity while protecting new growth. Many places, injection of Potassium phosphite into oak trees to help prevent infection with the Sudden Oak Death caused by the pathogen Phytophthora.
Agarwood trees could be injected with diluted potassium phosphite @ 5% concentration. The flow of the chemical as it is introduced through the trunk. The chemical reaches every element of the tree including the fine root hairs. Tree injection is also an economical alternative to other methods. The tree receives the chemical ‘directly’ therefore wastage is minimized.
Other related info about disease Phytophthora…
Take away of the story:
Since development and spread of Phytophthora diseases are favored by wet conditions and curative action is difficult, some say there is no cure for phytophthora infections. If the trees are survive, their growth are distorted because of bad root development by fungal infections. So, avoiding to plant the trees in lower landscape is a first priority to consider.
The result is fatal, photo taken 2 months later