Mark Boudreau, Green Agroecology
A susceptible host may be present, and a virulent pathogen may be right where it needs to be to infect it, but no disease will ensue unless the environment is accommodating. This corner of the disease triangle (the Environment) is key to the development of a serious epidemic. Knowing how to manipulate the environment to discourage the pathogen is one of the grower’s best tools to prevent or minimize disease.
When environmental conditions are unfavorable for disease initiation (e.g. low humidity reduces the percent germination of fungal spores present), it is the equivalent of reducing the amount of initial inoculum for an epidemic. This tends to delay disease onset for pathogens that have multiple infection cycles in a season. However, after infection, changes in the environment will alter the rate of disease development.
The Key Elements
Although many aspects of the environment influence disease initiation and development, two of the most important factors are temperature and moisture. We discuss each of these below.
Every stage of a pathogen’s life cycle is affected by temperature, from spore germination to penetration to growth in the host to new sporulation and spore removal. There is a minimum, optimum, and maximum temperature which is different for different pathogens and even for each of these processes in any given pathogen. Thus, Verticillium dahliae is most active causing wilt at temperatures between 25-28ºC, but Verticillium albo-atrum will dominate at temperatures between 20-25ºC. Early blight of tomatoes is favored by warm temperatures, while late blight prefers cool conditions (thus making late blight often occur earlier!).
Water is essential for life, and generally moist conditions will favor the pathogen and disease development. In some cases free, liquid water is necessary for a process, such as spore germination. Or perhaps only high humidity is required for a life cycle stage. Bacteria ooze out of openings and fungi sporulate when humidity is very high, providing inoculum for new hosts. Water in one of its forms is often very important for pathogen dispersal, from rain splash of the ascospores of Venturia inaequalis (cause of apple scab) to the long-distance transport of Phytophthora sporangia in streams. The single most important factor encouraging seed and root rots, such as those caused by Pythium spp., is poorly-drained soil. Ironically, some soil-borne diseases (e.g. those caused by Sclerotinia spp.) can actually be controlled by temporarily flooding the soil.
The Critical Combination
While keeping the systems concept in mind, consider that temperature and moisture are best considered together because of the interactions that occur between them to affect disease development. Many studies have looked at pathogen growth or disease levels under different combinations of these two factors, including late blight (Mackenzie, 1981), bean rust (Berger et al., 1995), downy mildew of lettuce (Scherm & van Bruggen, 1995), and apple scab (Mills, 1994). Indeed, disease forecasting systems that advise growers on whether to apply fungicides or not, such as BLITECAST, TOMCAST, and the Mill’s Period, are based almost entirely on temperature and moisture.
Avoiding a Disease-conducive Environment
The first step in maintaining a healthy environment for plants, one that discourages disease, is to know the farm, keep its history and nuance in mind, and think of the big picture; in other words, maintain a systems approach. This involves a lot of long-term thinking, and we will summarize some of these strategies first.
The elements of a site which will affect disease are numerous, and they interact with the particular crop and management strategy being considered. We will look at the most important of these and consider how site selection can be used to minimize disease in an organic situation.
• Soil Drainage
Many diseases, particularly root rots and damping off caused by fungi like Pythium spp., are favored by heavy, poorly-drained soils. Some of these soils with a history of infestation simply cannot be used to grow most crops without installing drainage tile. This is one of the many elements of soil management that affects disease.
• Air Drainage
Low-lying valleys and coves may be subject to atypically cold conditions due to the downward movement of cold, dense air, making establishment of frost-sensitive crops risky, particularly in orchards and other perennial plantings that require high initial investment. In addition to frost pockets, these sites may have less air movement and higher humidity levels, also favoring disease.
• Elevation and Aspect
Elevation will directly influence the temperature and rainfall, in turn affecting disease. Likewise, the direction which a site faces in a mountainous or hilly environment, its aspect, may determine hours and directness of sunlight and temperature, both of which alter moisture conditions.
Besides considerations related to erosion and equipment use, slope affects infiltration and runoff of water and therefore soil borne pathogens, and can influence transport of inoculum such as the spores of Phytophthora spp., which will migrate down slope in flowing water.
Some sites may be protected by a windbreak of trees, hills, buildings, etc. that limit air movement and radiation exchange with the sky. These areas will experience higher humidity and more moderate temperatures than exposed sites, which are more likely to experience large fluctuations in temperature, including frost damage under still conditions, and strong winds.
• Disease History
Many sites simply cannot support particular crops, at least for a time, because of the persistence of a disease which may afflict that species. Cole crops must not be planted into soil which harbored clubroot of crucifers in the past; in other cases a suitable rotation will sanitize the soil after a time, or lower initial inoculum levels below the economic threshold.
An organic certifier will be particularly wary of what a grower’s conventional neighbor is spraying, and the grower also should be conscious of neighboring crops, alternate hosts of diseases, and insect vectors which may bring a “pathogen present” to your doorstep from your neighbor.
• Crop and Cultivar
A farmer’s overall rotation, including cover crops, greatly impacts disease in many ways. The species and variety chosen will have varying levels of resistance, but also directly affect the environment and so disease. Dense canopies retain high humidity and lower leaf and soil surface temperatures during the day, and reduce air movement. In most cases this encourages pathogens. Root exudates and decomposition products of sloughed-off roots and abscised leaves will influence soil microbial life to perhaps discourage pathogens (often by encouraging beneficial competing microbes) or in some cases to encourage disease organisms. Erect vs. prostrate forms of growth affect soil temperature and the potential for inoculation directly from the soil.
• Cultivation Practice
An erect growth form can be “created” by, for instance, staking. Tomatoes produced in this way are much less likely to experience blight than those allowed to trail. Many consequences can accrue from conservation tillage, such as cooler soil temperatures for longer periods in the spring due to residue. High tunnels and greenhouses also create entirely distinct environments which could either encourage or discourage particular diseases, but overall allow much greater control for the grower.
In this unique type of cultivation, neighboring plants can have a profound effect on the microenvironment of the pathogen, sometimes in unexpected ways. For instance, one would expect a low-growing monocropped bean to be warmer on a sunny day than one intercropped with corn, which would provide shade. This would be expected to favor most fungi on the bean. However, on a clear night the monocropped plant would radiate heat into the sky, making it cool and perhaps leading to dew formation, whereas an intercropped bean may be more protected and remain warmer and dew-free. This would discourage most fungal infections.
Once long-term strategies have been put in place to help prevent disease from getting a foothold on the farm, season-to-season decisions are made which influence the environment and, in turn, disease. These medium-term techniques are considered next.
Exactly when planting or transplanting is done can be used to avoid disease. A grower must be familiar with the optimal conditions for disease development and the local climate to take advantage of this option.
• Generally, seed should be planted when temperature and moisture are optimum for rapid germination and growth. Seed rot and damping off are more probable, especially for untreated organic seed, under wet and cool conditions where germination and growth are delayed.
• Late planting can help control barley yellow dwarf in rice, Fusarium root rots of beans, downy mildew of sunflower, and stalk rots of corn. In Florida, late planting of winter tomatoes (end of November) means cooler weather, and this helps control Fusarium wilt.
• Early planting can help reduce diseases such as downy mildew and leaf blights of corn, powdery mildew of peas, and common smut and fungal root rots of wheat. Note that conflicts may arise and planting time must be adjusted to a grower’s particular location and disease pressures. Fusarium root rot of peas is favored by early planting, but powdery mildew may be controlled by early planting.
• Celery, with one of the most intense fungicide schedules of any vegetable primarily for control of Cercospora blight, should be planted at the coolest time practical to deter this warm-season disease. Planting date also can be adjusted to manage some abiotic diseases.
Density and Pattern
The canopy environment is affected significantly by the arrangement of plants and between- and within-row spacing. For example, in a study by Rotem & Ben-Joseph (1970), dense plantings of potatoes had more than seven times the incidence of late blight in mid-June as compared to a sparse planting. Read more about this topic in The Host article.
Manipulating the environment to avoid disease does not stop during the season, despite all the essential long- and medium-term steps that have been taken. Every day when a farmer ventures into the field, decisions are made and actions taken that can profoundly affect the possibility of yield-threatening disease. We will consider these decisions last.
• Manage water
Keep the foliage dry and soil not too wet through appropriately-applied and well-timed irrigation. For example, drip irrigation will help prevent spread of strawberry anthracnose by limiting splash dispersal of spores. Manually removing water is even a possibility–dollar spot on turf grass can be controlled by dragging a hose or pole across grass to remove dew!
Avoid working in the field when conditions are wet. Pathogens will spread easily, and soil will be compacted, which decreases aeration and drainage and thus favors disease.
• Manage fertility
Nutrient addition at the wrong time might encourage certain diseases. Nitrogen can foster excessive vegetative growth that is good news for most rusts, powdery mildews, and fireblight of apples, for instance.
Alternatively, the right nutrient available at the right time, best attained through balanced long-term fertility management, generally helps the host avoid infection (though it is a myth that all pathogens prefer plants in poor health). Nutrients for vigorous early growth allow a plant to “grow out” of its susceptibility to damping off and seedling rots.
Material covering the ground on which your plants are growing can help with disease, but in some cases encourage it. Organic mulches can promote beneficial microbes and encourage disease-suppressive soils. They can also hinder dispersal of rain-splashed inoculum, while plastic mulches can exacerbate splash dispersal.
Timing is important. Early in the season, mulch can encourage damping off by keeping soil from warming and drying, especially problematic for disease-susceptible young stems. But later in the year, fruits such as tomatoes or cucurbits can be kept out of contact with soil and avoid rot thanks to intervening with mulch.
The strategy for disease control which should be considered a last resort is application of chemicals. Not only must organic farmers select from a limited list of allowed pesticides, but they must be chosen and applied with care. We can consider these chemicals as altering the pathogen environment to make it unfavorable, but we deal with it in more detail in other articles on the pathogen component of the disease triangle, and what products and inputs can be used on organic farms.
References and Citations
- Agrios, G. 2005. Plant Pathology. 5th ed. Academic Press, NY.
- Berger, R.D., B. Hau, G. E. Weber, L. M. A. Bacchi, A. Bergamin Filho, and L. Amorim. 1995. A simulation model to describe epidemics of rust of Phaseolus beans I: Development of the model and sensitivity analysis. Phytopathology 85: 715-721.
- Jones, A. L. 1986. Role of wet periods in predicting plant disease. p.87-100. In K. J. Leonard and W. E. Fry (eds.) Plant Disease Epidemiology 1: Population dynamics and management. Macmillan, NY.
- McCartney, H. A. 1997. The influence of environment on the development and control of disease. p. 3-32. In N. A. Rechcigl and J. E. Rechcigl (eds.) Environmentally safe approaches to crop disease control. CRC Lewis, Boca Raton, FL.
- Mackenzie, D. R. 1981. Scheduling fungicide applications for potato late blight with BLITECAST. Plant Disease 65: 394-399.
- Mills, W. D. 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. Cornell Extension Bulletin: 630 Ithaca, NY.
- Rotem, J., and J. Ben-Joseph. 1970. Evaporation rate as an indicator for potato late blight development in plots of different foliage density. Plant Disease Reporter 54: 768-771.
- Scherm, H., and A. H. C. van Bruggen. 1995. Concurrent spore release and infection of lettuce by Bremia lactucae during mornings with prolonged leaf wetness. Phytopathology 85: 552-555.