No matter what crop you’re growing, it’s likely to be susceptible to fungal pathogens. Conventional and organic growers alike seek to protect their crops from these destructive pathogens. Although synthetic chemicals have become a primary tool in many growers’ toolboxes, bio-based products are rapidly gaining ground as eco-friendly, safe and effective means of combating fungal diseases.
No matter how you plan on attacking a fungus, gaining an understanding of the fungus lifecycle, and how to prevent these pathogens from completing it, is prudent. Pathogens that cause plant disease thrive under conditions conducive to their needs, and making things uncomfortable for them is the best place to start.
“We want to maximize performance. That means getting all of the disease control without developing resistance to that fungicide,” Bond McInnes, Ph.D., of AgBiome said. “How do you maximize the best use of that fungicide?”
McInnes has spent his career working in plant pathology. He spoke during a recent webinar on spring plant diseases, addressing the proper management of fungal diseases.
Before any planting occurs, McInnes recommends developing a fungicide program. This involves understanding the fungal lifecycle, knowing the susceptibility of your crop to fungal diseases and taking a targeted approach using not only traditional fungicides but also biological options.
Plant diseases develop when a pathogen meets a susceptible host and environmental conditions are right for the pathogen to grow, reproduce and infect the host plant. Most fungal pathogens disperse via spores, some of which can live for many years in soils before finding and invading a host. Weather conditions – humidity, amount of rainfall, temperature and soil moisture – all impact the fungal pathogen’s ability to complete its lifecycle, of which the spore phase is one component.
Spores are the primary means of fungal dispersal. Spores can move via rain, wind, air, equipment and people. Spore germination and tube germ growth occur under proper environmental conditions. This pre-infection stage, before the plant is colonized, is the best shot at preventing fungal issues in crops, McInnes said.
Once the fungus enters the plant via root or tissue contact, it gains the nutrients it needs to rapidly multiply. The fungus then disperses itself into the soil or air, beginning its cycle again.
The amount of spores impact the severity of the disease, as does the host’s resistance to the disease. Many fungal pathogens can have multiple lifecycles during a single growing season, compounding disease prevalence and severity in crops.
“Pre-planning so you have a fungicide applied prior to disease development, or at the very first sign, is where you get the maximum use,” McInnes said. After colonization, there is a short window to stop disease progression, and not all fungicides are effective post-exposure.
All fungicides are preventative and can be applied prior to infection. The residual amount of protection following application varies by product as well as with environmental conditions. Uniform spray coverage is imperative for preventative fungicides to work.
Curative fungicides are those that can be effective after the pathogen has entered the plant but prior to development of symptoms indicating plant infection. Eradicant fungicides work after the symptoms are present, and must reduce lesions or inhibit spore viability or production to be effective. Antisporulant fungicides do not have to enter a plant and work to inhibit spores but may or may not attack lesions.
If you apply a preventative fungicide but still have disease issues, it does not necessarily mean the fungicide was ineffective or that resistance has developed, McInnes said. Proper sprayer adjustments and applying fungicides according to label instructions are important.
Pathogens rarely develop resistance if products are properly selected and applied. Fungicide mode of action, as outlined in the FRAC classification codes, should be used to “alternate modes of action to get disease control and prevent resistance development.” FRAC codes are indicated on each fungicide label, and should guide farmers in selecting products for control while eliminating resistance risk.
Soil-borne fungi have low risk of developing resistance. Others, such as foliar pathogens which cause mildews, are at a high risk of becoming resistant to fungicides. Resistance can be qualitative (where it rapidly develops) or quantitative (developing gradually in population), and is influenced by fungicide mode of action. In most fields, quantitative resistance occurs as resistant pathogens survive year-to-year, and each year more of them are present in the field.
Mixing and matching fungicides in a tank mix or pre-mix involves selecting ones with differing modes of action. Both must also be equally active at the same effective rate, McInnes said. This can assist with reducing resistance risk.
Fungicides work on the plant in various ways – protectant, penetrant, systemic or vapor-phase. Protectant fungicides work on contact and can move externally on the plant surface via surface water. Penetrants move into treated tissues, and also move from treated to untreated surfaces. Systemic fungicides move through plant tissue from treated to untreated areas, either locally (within a leaf) or throughout the plant. Vapor-phase products move from treated to untreated areas due to their volatility, as they move outside of the plant in a vapor stage.
Single-site fungicides, which target specific organisms, used after broad spectrum fungicides are utilized is recommended. Matching the product to the crop and to the pathogen, following label directions and paying attention to pre-harvest intervals and re-entry intervals are other important factors in fungicide selection and usage.
Scouting for and properly identifying pathogens is required for effective fungicide use. Crop rotation and keeping crop debris off of fields, to prevent overwintering spores from finding a suitable host, are needed. Using resistant cultivars as much as possible, and applying fungicides early in the season, not after infection has occurred, will all help reduce resistance issues.
Today, newer fungicides using biological controls are becoming more important to growers. These biologicals are microbial or plant-based, have various modes of action, have little concern with pre-harvest and re-entry intervals and are flexible in their application. Consumer demands for transparency in food production, concerns about worker and environmental safety and compatibility with integrated pest management strategies make biological fungicides effective, safe and accepted.
Biochemical fungicides include oils and clays, pheromones, plant extracts and insect growth regulators. Microbials, which are primarily bacteria but can also be fungi, viruses, yeasts or protozoa, work by attacking disease-causing pathogens in various ways. They may produce compounds harmful to fungal pathogens, or colonize the plant and prevent pathogens from doing so. They may trigger plants to fight disease, or promote plant tolerance to infection. Biofungicides can be broad spectrum, and are useful against soil and foliar pathogens.
“The microbials are really dominating the market,” McInnes said. “These are some new, exciting fungicides that are being introduced to growers – and can really help you in your production.”
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