Damping off and root rots - Pythium

Key characteristics

Infected seeds and seedling roots become water-soaked and quickly decompose.
Seedlings displaying symptoms soon after emergence quickly die.
The disease is prevalent in seed raising beds with excess moisture.

Introduction

There are many Pythium species with varying degrees of host specificity and pathogenicity. Diseases can be expressed as seed decay, pre- or post-emergence damping off and infection of the roots or stems of young plants. The pathogen is inclined to moist soils typical of those subjected to excess irrigation, poor drainage and very high humidity. Little research has been done on this disease in Vietnam, however there is a high potential for this pathogen to cause economic losses here.

Symptoms

Damping off of seedlings is expressed as cotyledon and leaf chlorosis, then a watery rot appears in the taproot and hypocotyl at or near the soil line. When the roots decompose, the stele is left intact to leave only a white strand, which is followed by seedling death. On soybean seedlings P. ultimum Trow causes a wet rot, where P. debaryanum Hesse causes small, black dry lesions on the cotyledons. Plants with damaged root systems may continue to grow, but can appear stunted to varying degrees.

When onion plants are infected after the seedling stage they are stunted and the leaves yellow from tip to base.

In root rot of mature plants, feeder roots die, then lesions up to 2 cm long develop on the lateral roots. The size of the lesions increase, the plant shows aboveground symptoms of wilt, chlorosis and necrosis and the disease then spreads along the runners. In cucurbit root rots, fruit is then exposed to sunburn and the quality is reduced.

P. aphanidermatum (Edson) Fitzp. causes a rot of maize stalks at the internode just above the soil line at the time of tasselling. The diseased area is brown, soft, wet and collapsed. The stalks may be twisted. Plants may remain green for a few weeks after being infected, as the vascular tissue is not affected.
A watery wound rot of potatoes occurs upon invasion of wound sites by P. ultimum and P. debaryanum. Infected tissue is spongy, wet, discoloured and may have cavities.

The Pathogen

Pythium belongs to the order Peronosporales within the class Oomycetes. Pythium produces a white, fast-growing mycelium , which produces sporangia. The sporangia can germinate directly by producing one to several germ tubes, or hyphae with vesicles at the end form. From the vesicles, 100 or more zoospores are released, they form cysts and then germination occurs. The germ tubes that are produced upon germination can penetrate host tissue to initiate infection or produce another vesicle to continue the zoospore life cycle. Club-shaped antheridia produced in the mycelium develop germ tubes which enter the spherical oogonia and fertilisation occurs. The wall of the oogonium thickens to form an oospore. From the oospores come sporangia and the cycle repeats.

Distribution within Vietnam

This disease is present in all parts of Vietnam.

Host Range

P. ultimum, P. aphanidermatum, P. myriotylum Drechs., P. debaryanum and P. irregulare Buisman cause damping off and root rot diseases. Those hosts susceptible to Pythium root rot include cucurbits (melon, squash, cucumber, watermelon and pumpkin), onion, lettuce, beans, mungbeans, soybeans, maize, potatoes, coffee, pineapple and sugarcane. The seedlings of many plant species are susceptible to damping off.

Epidemiology

The Pythium species that infect these plants are ubiquitous in soil. They will cause a disease because environmental conditions are favourable, not as a result of the spread of the pathogen into a new area. Water movement, through irrigation or rain splash can however disperse active zoospores. Plants are generally most susceptible to Pythium when the conditions are unfavourable for plant growth such as unfavourable temperature, excessive moisture, low light or poor nutrient availability.
Each Pythium species is favoured by different conditions. P. ultimum and P. debaryanum inhabit cool (10-15°C), moist soil as saprophytes on crop residues. Other Pythium root rots, such as those caused by P. aphanidermatum, P. irregulare, P. sylvaticum and P. myriotylum occur in warm (25-36°C), moist soil. Moist soil is conducive to disease development because the low oxygen level encourages the release of exudates from seeds. These exudates stimulate the growth of Pythium species towards the emerging plant tissue. As seedlings continue to grow, the risk of plant death from post-emergence damping off decreases.
Pythium survives in the soil by saprophytic growth or by forming resting spores such as oospores, hyphal swellings, chlamydospores and encysted zoospores. The spores are stimulated to germinate by nutrients such as those in seed and root exudates.

Control

Soil with large populations of fungi and other organisms suppress both the saprophytic and pathogenic activity of Pythium species. The addition of organic matter may contribute to soil microbial populations.

Minimising periods of excessive soil moisture are essential for controlling Pythium diseases. Improve the drainage of the seed raising beds by planting on raised beds and if symptoms are present only irrigate for short periods on alternate plant beds to maintain plant growth.

Pythium damping off

Root rot symptoms on lettuce

P. ultimum culture shot

Photos: Len Tesoriero

Botanic Gardens Trust, Sydney, Australia

Science