Pythium spp. (Cotton)

Scientific Name Pythium ultimum Trow., Pythium aphanidermatum (Edson) Fitzp., Pythium irregulare Buisman, Pythium sylvaticum W.A. Campbell & J.W. Hendrix
Common Names English: Seedling disease, (pre-emergence) damping-off, seed rot; German: Umfallkrankheit, Wurzelfäule; Spanish: Caída de almácigo; French: Fonte des semis
Description Pythium species are members of the division stramenopiles (chromista), class oomycetes (water molds), family pythiaceae. The thick-walled oospores (sexual spores produced by the union of oogonium and antheridium) resistant to desiccation are the primary survival structures of Pythium species. Oospores germinate from 15 to 35 °C and pH 5.2 to 8.6. Temperature optimum for growth of P. aphanidermatum is 34 °C, with a minimum at 10 °C and a maximum above 43 °C.
On artificial media cottony white aerial mycelium is produced with aseptate hyphae 7 to 10 µm in diameter. In P. aphanidermatum zoosporangia are formed terminally, spherical oogonia (20 - 25 µm) also terminal, sack-shaped antheridia (10 - 14 µm) generally intercalary. Oospores are aplerotic, 20 to 22 µm in diameter, with an oospore wall 1 to 2 µm thick. Site of formation of sporangia, oogonia and antheridia as well as their shape and dimensions slightly varies for the other Pythium species.

Biology

Damage

As part of a pathogen complex including also species of the genera Thanatephorus, Thielaviopsis and Fusarium, Pythium species can attack the seed before or at germination. The pathogen(s) causing seedling disease result(s) in pre- and post-emergence damping off. Early infection causes seed decay and pre- and post-emergence damping-off. Symptoms include seed decay and seedling decay before emergence, girdling of the emerged seedling stems, and root rot often associated with dark necrosis and water-soaked appearance of roots and hypocotyls. The disease is associated with a soft, watery rot.
Pythium spp. usually attack the seed and below-ground parts of young seedlings, while Thanatephorus cucumeris usually causes sunken lesions on the hypocotyls (sore shin). As a number of pathogens cause similar symptoms, pathogen isolation and identification are needed for diagnosis.
Seedling disease results in uneven, slow-growing stands; in some years, replanting is necessary.

Lifecycle

Sporangia and oospores of P. aphanidermatum germinate directly by producing a germ tube or indirectly by producing zoospores. They are attracted primarily to the region of root elongation, root hair emergence or the root cap where they encyst and penetrate the host. Under non-favourable conditions, zoospores may encyst in the soil, remaining viable as long as soil moisture content and temperature are suitable. The response of zoospores to root exudates components differ among Pythium species. Host tissue is penetrated either with or without appressorium formation. Pythium spp. commonly reproduce within diseased tissue, but are also good saprophytes colonizing organic material in the soil.

Occurrence

Additional Crop Information

The host range of Pythium species is very wide including cereals as well as dicotyledonous species.

Agricultural Importance

Under low-temperature and high-moisture conditions in the soil, P. ultimum and P. irregulare are the most damaging species to cotton seed and seedlings, P. aphanidermatum is favoured by higher soil moisture content and warmer temperatures typically for the tropics.
Transmission of Pythium species is normally associated with movement of infested soil or contaminated plant material.

Control

Useful non-chemical contribution to Integrated Weed Management

Seedling disease is favored by cool, wet conditions and seems to be more prevalent on sandy, low-organic-matter soils. Deep-planting, poor seed bed conditions, compacted soil, nematode or insect infestations, and soil-herbicides like dinitroanilines may increase disease probability.
As the level of genetic resistance to seedling disease is low, the use of high-quality seed and planting at higher soil temperatures, and planting on raised seedbeds are recommended.

Chemical Control

Broad spectrum fungicidal seed treatments for use in dicotyledonous field crops such as cotton often include (beside SBI or carboxamide fungicides for basidiomycete and ascomycete control) also an anti-oomycete partner for the control of Pythium and Phytophthora species causing damping off symptoms. Therefore, seed treatments or other early applications (planter box, in-furrow treatment) with either QoI (=strobilurin) fungicides, phenylamides or etridiazol are principally well suited to reduce significantly seedling diseases caused by Pythium species.

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