Drought Effects Upon Plant Disease Potential
Disease potential and severity will vary depending upon the pathogen and the crop.
Plants and plant pathogens respond differently to environmental conditions and variations that occur throughout their life cycles. Temperature and moisture extremes create stress on plants, and in some cases predispose plants to greater loss from specific types of plant pathogens. On the other hand, temperature and moisture extremes can negatively impact specific types of plant pathogens, and reduce their ability to survive and cause plant diseases in stressed plants (Plant Pathology, 4th ed., G. N. Agrios, Academic Press).
Moisture Effects on Plant Pathogens
Moisture is indispensable for the germination of fungal spores and penetration
of host tissues, and for the activation of bacterial and nematode pathogens
before they can infect a plant. Moisture (especially in the form of rain
or irrigation water) is critical for the distribution and spread of pathogens
from plant to plant, and field to field. Access to abundant moisture increases
the succulence of host plants and thus their susceptibility to certain
pathogens, which in turn affects the extent and severity of diseases.
The occurrence of many diseases in a particular region is often closely correlated with the amount and distribution of rainfall or other sources of moisture (irrigation water, dew period, relative humidity) during the plant and pathogen cycles. Indeed, rainfall and other moisture sources such as irrigation water may determine not only the severity of the disease, but also whether the disease will even occur in a given season.
Some foliar-infecting pathogens, such as those causing late blight of potato or downy mildew of onion must have high relative humidity or free moisture in the environment throughout their development. Other foliar-infecting pathogens, such as that causing powdery mildew, are actually favored by lower moisture (especially relative humidity less than 70 percent) and are inhibited by high moisture conditions.
Some soil-borne pathogens such as Pythium damping-off of seedlings are favored by high moisture, and the severity of the disease is proportional to the amount of soil moisture. The increased moisture favors movement of spores in soil water films, and plant roots may be more stressed for oxygen due to waterlogged and cooler soils. Other soil-borne pathogens such as Rhizoctonia or Sclerotinia cause the most damage on wet, but not flooded or dry soils. Other soil-borne pathogens such as Fusarium grow fairly well in dry soil environments on plants that are stressed by insufficient water and/or high temperature.
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Temperature Effects on Plant Pathogens
Most plants grow best within a temperature range of 60º – 86º
F (15º – 30º C), as do most plant pathogens. Pathogens
are less active and cause less disease during prolonged periods of extreme
temperatures; that is less than 60º F or greater than 86º F.
Plants are generally injured faster and to a greater extent when temperatures
become higher than the maximum for growth than when they are lower than
the minimum. High temperature often causes its effects on the plant in
conjunction with the effects of other environmental factors, particularly
excessive light, drought, lack of oxygen, or high winds accompanied by
low relative humidity. High temperature can be responsible for sunscald
injuries on the exposed side of plant tissues. High soil temperature at
the soil line sometimes kills young seedlings, or causes cankers at the
crown on stems of older plants.
Drought Effects on Plant Diseases
Insufficient moisture and high temperature are drought conditions that
negatively affect plant survival, growth and reproduction. Abiotic responses
in stressed plants can include stunting, light color (pale green to light
yellow), few and small leaves, few and small pods or fruits, wilting and
death. Plants weakened by drought are also more susceptible to some plant
pathogens and insect pests.
Soil-borne pathogens such as Fusarium (Fusarium root rot and Fusarium wilt of dry bean) and Macrophomina (ashy stem blight of dry bean, charcoal rot of corn) are usually favored by drought conditions that stress plant roots and predispose them to colonization and infection. Foliar pathogens such as powdery mildew (of sugar beet, pumpkin) are also favored by extended periods of low moisture and relative humidity. Drought conditions discourage the survival and activity of other soil-borne pathogens and most foliar-infecting pathogens that require high moisture conditions for survival, infection and disease development.
So, continued drought conditions in Colorado and the surrounding region will discourage the survival and outbreaks of most foliar-infecting pathogens such as rust or bacterial blight. However, soil-borne pathogens such as Fusarium and Macrophomina will be favored by these types of conditions that stress the plant. So, disease management should target any cultural practice that reduces stress to the roots such as planting during recommended periods when soil temperatures and moisture are adequate for emergence, and early-season tillage to remove soil compaction and permit more vigorous root development. And, always follow an integrated pest management approach that targets all pests (abiotic or biotic) that can threaten plants and their productivity. The following dry bean IPM strategy can be adapted to other crops and plant species grown in our region.
SUMMARY OF Dry Bean Disease IPM STATEGY for 2003:- Rotate out of dry beans for at least 2 years to reduce survival of soil-borne and foliar-infecting pathogens and other pests.
- Eliminate bean debris and sources of volunteer beans during the fall of 2002 and spring of 2003.
- Plant high quality, certified, treated seed of disease resistant varieties, if available and suitable for your market needs, when soil moisture is adequate and soil temperatures are greater than 60º F at the seeding depth.
- Follow recommended production practices to avoid stress from extremes of moisture, temperature, and soil compaction.
- Manage water and fertilizer inputs to provide adequate, but not elevated components to avoid excess canopy development.
- Carefully scout fields to detect foliar infection as early as possible, get confirmation of disease diagnosis from appropriate experts in your area.
- Monitor reports on weather patterns, disease forecasts, and confirmed pest sightings in your region via the CSU VegNet at http://www.colostate.edu/Orgs/VegNet/
- When and if infection is confirmed in or near your field, implement a timely program of fungicides and bactericides with protectant and systemic modes of action. Rotate appropriate fungicide chemistry, apply labeled rates, use an adjuvant, and stay within recommended spray intervals.
- Adjust combine at harvest to maximize seed quality, and reduce seed loss that can produce volunteer plants next spring.
- Thoroughly incorporate diseased crop residue to reduce carryover and potential disease pressure the following season. Rely upon cultivation and herbicide in next year’s rotation crop to reduce volunteer bean emergence and possible infection by foliar-infecting pathogens which can then be spread to next year’s host crop.
