Pythium stalk rot is caused by the oomycete Pythium aphanidermatum. The disease is prevalent in some hot and humid tropical and subtropical zones and in some temperate areas. The disease is more prevalent in wet areas and in poorly drained soils. P.aphanidermatum has a very wide host range and is known to cause root and stalk rots in a range of cereal and horticultural crops.
Previously Pythium butleri was also considered a causal agent, but the two species have since been made synonymous.
Pythium stalk rot is usually confined to the first internode above the soil. Stalks become soft, water soaked, darkened, and collapsed. Frequently the stalk at the site of infection becomes twisted as the stalk tissue rots, eventually leading to stalk lodging. Stalks do not usually break off completely. Infected plants may appear healthy for extended periods as vascular bundles often remain intact for some time. Unlike other stalk rots, Pythium stalk rot can occur prior to flowering.
Hyphae of P. aphanidermatum are non-septate and hyaline. Sporangia appear inflated and measure 4-20 × 50-1,000µm. Sporangia can be branched or unbranched and can form complexes. Zoospores are kidney shaped, laterally biciliate, and measure about 7.5 × 12µm. Oogonium are spherical (22-27µm in diameter) and terminal while antheridia are often intercallary. Oospores are thick walled and 17 to 19µm in diameter. Typically oospores will be observed from diseased stalk tissue.
Isolation in culture media may be necessary to distinguish P. aphanidermatum from other organisms that also cause stalk rots.
Why and where it occurs
Pythium stalk rot is more prevalent in areas where water and air drainage is poor and humidity is high. The disease occurs in depressed areas of the field or in river bottom fields that are characterized by wet soils. High temperatures (25-35°C), high crop density, and high levels of nitrogen fertilizer all favor disease severity.
Figure 1. Geographic distribution of Pythium
Pythium aphanidermatum is known to infect a wide range of cultivated crops, including cereals and grasses, cucurbits, horticultural crops, and cotton. P. aphanidermatum is also an important greenhouse pathogen.
P. aphanidermatum survives in the soil as mycelium or oospores. Oospores can survive in the soil for several years. They produce sporangia which release motile zoospores. Zoopores swim towards plant or root tissue in response to exudates which serve as nutrient or chemotrophic stimulants. Zoospores infect the plant directly at or just below the soil surface where free water is present, leading to the development of characteristic lesions. Mycelium within the infected host tissue gives rise to oogonium and antheridium, the sexual stage of the life cycle. The antheridium fertilizes the oognium producing oospore, which is able to overwinter in crop debris or in the soil. The pathogen is dispersed with either the movement of infected crop debris or with flooding or excess wetness, which transports oospores and enables zoospores to swim freely.
- Mechanism of damage: Pythium stalk rot results in lodging of the plant. Plants are therefore killed prematurely, leading to yield loss. Often infection occurs prior to tasseling.
- When damage is important: Damage is most prevalent in warm and humid regions where soils are particularly wet. Valley areas and river bottom fields are particularly susceptible to infection. However dispersal of the pathogen by natural means is often limited and hence infection can be limited to particular areas of a field.
- Economic importance: Pythium stalk rot is generally confined to hot and humid regions where maize is cultivated in poorly drained soils. Damage is usually localized in a field. Occasionally entire parts of a field may lodge prematurely, although extensive damage in the field is rare as the pathogen dispersal by natural means is often limited.
- Ensuring that maize is planted in well drained soils will reduce the risk of Pythium stalk rot.
- Irrigation should be controlled to avoid water logging, which favors zoospore dispersal.
- Proper sanitation will ensure that infected crop debris does not overwinter.
- Increased air circulation within the field through reduced crop density can reduce disease incidence.
- In greenhouse environments and in fields where economically viable, solarization, fumigation and soil drenches (fungicides) have been used to reduce inoculum levels in the soil.
- Various biological control agents such as Gliocladium catenulatum and Trichoderma species are also being investigated for control of P. aphanidermatum. However, these treatments are designed as a soil drench for greenhouse environments.
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Contributor: Biswanath Das