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Karen Delahaut, Outreach Specialist, UW-Madison
Plant diseases are any harmful alteration in the physiology (function) or morphology (structure) of a plant. They may be biotic―caused by living organisms or abiotic and result from physical or chemical irritants. For any biotic disease to occur, three things must be present in close proximity for a prescribed amount of time. These components are a susceptible host plant, a virulent pathogen, and an environment conducive to disease development (infection). This is referred to as the disease triangle. If we look at each component individually, we can identify some tools for preventing disease from occurring in the first place by making a few modifications.
Let’s begin by looking at the environmental component. You can avoid many pathogenic diseases by simply modifying the environment in which the plants are grown. Proper sanitation is a key preventative strategy in not only preventing diseases but also insect outbreaks. Unpasteurized potting media, contaminated stock plants, crop debris in the greenhouse, and dirty benches and tools are just some of the sources of pathogens. Take care to practice good sanitation and you will do much to prevent disease problems.
The next thing to look at is the climatic conditions in the greenhouse. This includes temperature, humidity, and air and soil drainage. Condensation and high humidity, as well as overhead watering, can cause prolonged leaf wetness or raise soil moisture and provide the free water necessary for infection to occur. The humidity levels in dense plant canopies can be as much as 20% higher than that in the aisles. Spacing plants further apart or placing fans over the bench will facilitate air movement and dry the foliage more quickly. If you orient your plant rows so that the fan circulates air between the rows, you will further reduce the canopy humidity. Also, plants on benches have better air flow than those placed directly on the floor and benches with an open mesh construction will allow for air circulation above and below the bench as well as across the top.
Finally, be sure to provide the plants with the nutrients and water they need to thrive and remain healthy. It’s not just a matter of applying the right amount of fertilizer and irrigation water but also the timing, frequency, and method used so the plants are never under moisture or nutrient stress. Understand which stages of growth have higher nutrient and moisture demands and be sure to meet these demands to avoid stressing the plant.
The next leg of the disease triangle is that of the pathogen. The mere presence of a fungal spore or bacterial cell doesn’t mean that infection will occur. The pathogen must be in an infective state and the host plant in a receptive stage of growth or under stress. As with the environmental component, there are cultural practices you can employ to reduce the likelihood of disease including crop rotation, rouging out infected plants, soil disinfection, heat treatment, and sanitation.
It’s important not to plant the same crop in the same location season after season. Sometimes this is difficult due to limited space but breaking up the crop cycle and planting something different or leaving the greenhouse empty for a brief period in conjunction with good sanitation practices can prevent the build-up of disease organisms.
If you notice diseased plants during your scouting visits, remove any plants that are too far gone to be treated. This is particularly essential in the case of viruses as there is no cure once a plant is infected with a virus.
Using heat is a type of mechanical control that will kill seed-borne diseases, eliminate insects on bulbs and corms, and may prevent the spread of viruses from a stock plant to the cuttings. The timing and temperature of the treatment is critical so make sure you follow the directions for the particular disease and plant you are treating. Exposing the plant to temperatures that are too high or for too long can kill it while temperatures that are too low or for too short a duration will result in inadequate control. Heat can also be used to kill pathogens in recirculated or reclaimed water.
Pathogens can be excluded from the host plant by quarantining new plants or those known to be infected. Also, make sure you only purchase certified stock from reputable growers. It’s not cost effective to buy bargain basement stock from a nursery that’s going out of business if you find out the crop is contaminated and you either have to destroy it or spend a lot of money treating the infection.
Last, but not least, the host plant is the third component in the disease triangle. Cultural and physical practices that avoid plant stress will greatly reduce the risk of a disease outbreak. Stressors include temperature or moisture extremes, improper fertilization, insect outbreaks, poor water quality and excess soluble salts, pesticide injury, and direct physical damage to the plants.
Genetic host plant resistance is another excellent first line of defense in preventing plant diseases, particularly in the case of viruses. There is one caveat in this situation however and that is that your customers must want to purchase the resistant host plant for the aesthetic characteristics it possesses. Since you are growing crops for their appearance you have to make sure that if you choose a resistant variety, that it provides essentially the same aesthetic qualities as the susceptible variety that the consumer’s demand.
Finally, to protect healthy plant tissue from becoming infected when other, infected plants are in close proximity, you can use fungicides. Keep in mind that few fungicides are actually curative and will likely not provide much in the way of cleaning up infected plants but repeated use will protect new foliage from becoming infected.
