Pythium coloratum. [Descriptions of Fungi and Bacteria].

A description is provided for Pythium coloratum. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. DISEASES: Carrot cavity-spot, seedling damping-off. HOSTS: Apium graveolens, Daucus carota (Apiaceae); Zantedeschia aethiopica (Araceae); Lactuca sativa (Asteraceae); Brassica oleracea (Brassicaeae); Beta vulgaris (Chenopodiaceae); Cucumis sativus (Cucurbitaceae); Persea americana (Lauraceae); Allium cepa (Liliaceae s.l.); Pinus radiata (Pinaceae); Lycopersicon esculentum (Solanaceae). GEOGRAPHICAL DISTRIBUTION: NORTH AMERICA: Canada (British Columbia, Ontario), USA (New York). ASIA: Iran, Taiwan. AUSTRALASIA: Australia (South Australia, Western Australia), New Zealand. EUROPE: Great Britain. TRANSMISSION: Contaminated soil, organic matter (oospores) and water (sporangia).

Amendment of soil with lime or gypsum and its effect on cavity spot disease of carrots (Daucus carota L.) caused by Pythium coloratum

Summary. Glasshouse trials were conducted to examine the effect of lime or gypsum amendments on the development of cavity spot disease of carrots in soil artificially infested with Pythium coloratum. Each amendment was applied to soil from the field at 4000 or 8000 kg/ha. Lime at both application rates significantly (P<0.05) reduced the incidence of the disease. In contrast, both gypsum treatments had no effect on the incidence of cavity spot disease. There was no significant (P>0.05) difference in calcium concentration between carrot roots grown in unamended, lime- or gypsum-amended soil with or without the pathogen. Calcium did not appear to play a direct role in the reduction of cavity spot disease. Under the controlled conditions of this glasshouse trial, reduction in the incidence of cavity spot appeared to be related to the increase in soil pH associated with the application of lime. Field trials at the site of soil collection will confirm whether this mechanism is related to field reduction of the disease following liming.

Effects of Host Age on Development of Cavity Spot Disease of Carrots Caused by Pythium coloratum in Western Australia Khaled

Three experiments were conducted with Pythium coloratum Vaartaja, a causal agent of cavity spot disease of carrots in Western Australia, to study the relationships between host age, time of infection and development of cavity spot lesions. Pythium coloratum was isolated frequently from 3-6-week-old asymptomatic roots of carrots grown in soils infested naturally or artificially with the pathogen. Carrots grown in containers of soil artificially infested with P. coloratum, but not those in naturally infested field soil, developed cavity spot lesions after 6 weeks. Early infection of carrot seedlings at or before 3 weeks by P. coloratum in artificially infested soils followed by their transfer to pathogen-free soil was sufficient to cause cavity spot disease at the time of harvest (16 weeks). The disease levels in this treatment were not different from those transferred to P. coloratum-infested soil. There was no significant (P > 0.05) association between carrot age and the ability of P. coloratum to infect the roots and to cause cavity spot lesions at harvest.

Zoospore encystment pattern and germination on onion roots, and the colonization of hypodermal cells by Pythium coloratum

The behavior of the fungus Pythium coloratum Vaartaja on onion (Allium cepa L.) roots with a hypodermis of long and short cells was studied. Frequency of encystment adjacent to long versus short cells was compared in live and heat-killed roots. To determine whether the state of suberization of hypodermal cells was correlated with the number of zoospores encysting next to the cell, roots on which zoospores had encysted were stained with toluidine blue to quench most autofluorescence except that caused by lamellar suberin. Penetration of fluorescent long cells, nonfluorescent long cells, and nonfluorescent short cells was compared. Encystment of zoospores on the onion epidermis was more frequent adjacent to short cells of the hypodermis than could be expected for random encystment. More zoospores encysted next to short cells on heat-killed roots than on live roots. Encystment was more frequent over nonfluorescent long cells than over fluorescent long cells. Penetration by the fungus was more frequent into short cells and nonfluorescent long cells than into fluorescent long cells.