How Does Irrigation Influence the Presence and Severity of Diseases?

Michigan vegetable and fruit growers can use irrigation systems to fulfill crop water requirements while minimizing the risk of yield loss associated with drought conditions. However, growers should consider the quality, amount and timing of the irrigation that specific crops require at various growth stages.

Sources of irrigation water can harbor plant pathogens. In general, surface water such as rivers, creeks and ponds have a higher likelihood to be contaminated with plant pathogens when compared with water from sealed wells.

Watermolds such Pythium and Phytophthora species can be found in open sources of water and may be capable of causing plant disease. These two types of waterborne pathogens have spores (seeds of the pathogen) that are capable of swimming in water. In fact, at least 26 and 17 different species of Pythium and Phytophthora, respectively, have been documented in irrigation water globally according to “Plant Pathogens in Irrigation Water: Challenges and Opportunities.” These pathogens are well known to cause symptoms like damping off, such as in soybean damping off; foliar blights; and rots of different tissues like roots, crown and fruits (photos below).

Some plant pathogens can survive in the soil and can also be associated with cull piles of discarded and diseased produce. Surface water can be contaminated by runoff from contaminated fields according to “Characterization of Phytophthora capsici from Michigan Surface Irrigation Water.” By choosing a clean source of water, the risk of introducing plant pathogens carried by the irrigation water to your production fields is decreased.

Water not only plays a role in dispersing waterborne pathogens, but can provide the moisture that favors the infection and disease development of airborne pathogens. In order for disease to occur, a susceptible host, virulent pathogen and favorable environmental conditions are needed. Irrigation provides moisture and leaf wetness so that if spores of the pathogen have landed on a host plant, an irrigation event allows the infection process to begin. As the disease progresses, overhead irrigation can splash disperse pathogens from infected plants to nearby healthy plants, allowing the epidemic to continue to develop in the field.

To summarize, the key factors to consider when using irrigation water include:

• Quality. Use clean sources of water; well water is best.
• Amount. Pathogens are favored by water so do not apply more than what is needed by the crop for its growth stage.
• Timing. Consider current and forecasted environmental conditions and use irrigation scheduling tools.

Source: Michigan State University Extension