Phytophthora is a pathogen that does not discriminate from crop to crop and because of this, it is the number-one disease of nursery crops nationwide. Phytophthora causes millions of dollars in crop loss each year, partly due to being misdiagnosed or unrecognized, though a majority of crop loss is due to the lack of proper management and sanitation practices. For this reason, Phytophthora is known as the “the plant destroyer.”
Phytophthora is a water mold, therefore it thrives in a wet environment and allows the spores to travel and spread more easily. On top of that, the pathogen is good at reproducing. The spores move into the plant and root tissue under water logged conditions, and it is here that the pathogen is released into the plant and starts to reproduce.
Phytophthora is also great at hiding and its spores can lie dormant for long periods of time, making it difficult to know when an outbreak will occur. Phytophthora can survive in water, soil or any media within a growing environment.
There are more than 100 different varieties of Phytophthora and it is sometimes thought of as a fungus because it acts similarly, but the pathogen affects all plants differently. Typical signs of a Phytophthora infestation are yellow-vein chlorosis, suspicious spots or blotches on leaves, wilting brown-yellowing leaves, dropping of leaves, severe root loss, the presence of canker on root, stems and/or leaves and in severe cases, sudden plant mortality in patches. Some of these symptoms are identical to symptoms caused by natural conditions and other diseases, so it is easy to see the complexity of correctly identifying this disease.
One chemical or practice will not keep Phytophthora away, so how can growers take control of this pest? A program that focuses on the plant, the pathogen and the environment needs to be in place. To ensure all factors are covered, I have come up with an easy to follow program.
This first part is essential and often gets overlooked. It is critical to sanitize tools, equipment, structures and even people. Sanitation is important, but it should be noted that cleaning beforehand can significantly help decrease outbreaks. Chemicals shown to be effective for this are Peroxycompounds (hydrogen peroxide and peroxyacetic acid), percarbonates, copper and chlorine, though it is important to note some chemistries don’t kill spores and are harmful for the worker and the environment. It is also important for your sanitation chemicals to have minimal or zero REI to ensure no delays in production.
2. Contaminant-Free Growing Media
Growing media is frequently disregarded in nurseries and greenhouses, but since water and soil are the two major vectors in carrying Phytophthora, growing media needs to be addressed. Separation of growing media and cover is highly recommended and if possible, should be sterilized. There are many economical and effective sterilization chemicals that can be beneficial; one such is peroxycompounds. This chemistry has been deemed a viable alternative when compared to the bromide restrictions and limitations.
3. Water Treatment
It is known that water sources are huge breeding grounds for many pathogens including Phytophthora. Irrigation from ponds suffers from more pathogens than wells, but wells should also be treated. The biofilm within irrigation lines can be full of zoospores, which harbor disease. Treatment of the water as a curative to eliminate biofilm is highly effective, and it reduces spore counts. This is recommended at least once a quarter. Treating water with a maintenance program can also decrease the likelihood of infestation. Quarterly water tests are also recommended.
4. Chemical Rotation
Proper chemical rotation and implementation can significantly reduce the risk of disease. It is important to treat in rotation and treat proactively. Leaning on the same chemical can prove to be less effective and increases the chance of mutation and spread. It is also essential to use a contact, as well as a systemic chemistry, to ensure full environmental coverage. Tank mixing systemic chemistries with broad spectrum contact fungicides also increases the spectrum of activity, minimizing disease risk.
Documented data on spraying potassium phosphate in conjunction with peroxycompounds has been shown to be an effective spray program. Peroxycompounds and fluopicolide with potassium phosphate have also been proven to have results in control and prevention of Phytophthora and other greenhouse and nursery pests.
5. Insect Control
Insects and nematodes can be carriers of Phytophthora zoospores. A proper pesticide program includes instituting a tank mix or rotational program with an insect growth regulator. Azadirachtin-based insecticides can be used to suppress larval and nymph stages of insects, but it is important to look for products that have some anti-feedant capabilities to help suppress insects that have some contact with the plants. Not all azadirachtin products are the same. It is not recommended to use products with a glycogen base because over time, that can act as a food source for unwanted pests.
6. Plant Nutrition
Proper plant nutrition is important for a good defense against Phytophthora. It is essential to make sure to get the proper analysis of your fertilizer program and avoid salt-rich nutrient programs. An effective fertilizer program is balanced and ensures each specific crop is getting the correct amount of nutrients. A healthy immune system can help plants fight this battle. It may be time to reevaluate the current nutrition program or least double check that it matches the current crop’s specific needs and soil conditions.
Education is essential to a proper program for Phytophthora control. and implementing a concise program is the best place to start. Growers should check and see how many steps they follow and how many they can add. Checking off all is a daunting task, but starting is pertinent to effective pest control.