Plant pathogenic oomycetes


Oomycetes (water molds) are eukaryotic organisms in the group Stramenophiles, formerly considered Fungi such as ascomyctetes (various molds, yeasts) and basidiomycetes (mushrooms, rusts and others) to which they are morphologically similar but have no close phylogenetic relationship. Studies of metabolism, cell wall composition, and rRNA sequence indicate that oomycetes are more properly grouped with chrysophytes, diatoms, and brown algae. Many consider Oomycetes (Oomycota) a phylum in the kingdom Stramenopila, or alternatively, as Pseudofungi in the phylum Heterokonta of the kingdom Chromista.

Oomycetes are found in both fresh and salt water as well as in terrestrial environments. They produce flagellated, actively motile spores (zoospores) that are pathogenic to many crop plants and fishes.

Order Peronosporales contains a majority of oomycetes’ genera that are pathogenic to plants.



Phytophthora – Phytophthora infestans
Common name: potato late blight fungus

Organism Disease Transmission Management
Species of the genus Phytophthora are arguably the most destructive plant pathogens causing widespread damage to many horticultural and ornamental species, and to native ecosystems throughout the world. There are more than 65 Phytophthora species among plant pathogenic oomycetes. Phytophthora literally means plant destroyer, a name coined by Anton de Bary in 1861 for P. infestans after Irish potato famine. Infection begins as small light green to gray spots on leaves. In cool and moist conditions lesions expand rapidly to form large black rots (blights) that spread throughout the plant eventually killing it. Rots of the tuber also develop and are characterized by slightly sunken areas that have a brown to purplish skin. The aerial spread of inoculum is crucial to the epidemic phase of the pathogen. One or two cycles of transmission may occur in cut potato seed between cutting and emergence. Sporangia produced on tubers dispersed during handling and planting to healthy tubers used as seed. Such secondarily infected seed ar more likely to survive and produce infected stems than the original infected tubers. Treatment of seed with fungicides immediately after cutting reduces or prevents transmission of to healthy seed. Presprouting (chitting) of seed increases time during which the crop develops during the late blight low risk period and shortens the growing season during the high risk period. Also, affected tubers may be eliminated before planting. Resistant cultivar varieties are developed with unspecific or horizontal resistance of foliage and tubers.



Peronosporaceae – Peronosclerospora – Peronosclerospora sorghi
Common name: downy mildew fungus

Organism Disease Transmission Management
Produces ephemeral, asexual conidia and long-lived, sexual oospores. The production of oospores follows a monocyclic pattern, while conidia may be polycyclic. Infection with both oospores and conidia causes systemic disease. Causes destructive systemic disease of sorghum and maize, and can lead to 40-60% yield loss. Systemic disease causes sterility. Infected seedlings are chlorotic and stunted. Infected plants that survive the seedling stage produce a mixture of symptoms. Under cool, humid conditions a white downy growth of conidia and conidiophores is produced on the lower leaf surface. Oospores are produced in dying tissues in older leaves and are released when shredding occur. Infections are often caused by soil-borne oospores. As the season progresses, conidia are produced on liesions and infect healthy plants. The fungus overwinters as oospores in the soil, both free and contained in plant debris. The following measures help reduce the pathogen: seed treatment and a foliar spray with fungicides; breeding for resistance; crop rotation; tillage practices associated with crop rotation serve to bury inoculum in the soil and decrease its viability.



Peronosporaceae – Sclerophthora – Peronosclerospora rayssiae
Common name: brown stripe downy mildew

Organism Disease Transmission Management
The mycelium of S. rayssiae var. zeae is found in the leaf mesophyll; the hyphae are irregular in shape and are lobulate. Sporangia (measure 29-66.5 x 18.5-26 μm) are hyaline, ellipticoidal, smooth-walled, have a projecting apex and a peduncle. Four to eight zoospores are formed within a sporangium, which may encyst inside or outside the sporangium. Oogonia are found scattered in the leaf mesophyll or the substomatal cavities, never in the vascular bundles. Causes brown stripe downy mildew. Originally from India; potential bioterrorism weapon. Yield losses range from 20 to 90%. In early stages of infection leaves show narrow chlorotic or yellowish stripes, 3 to 7 mm wide; in some maize genotypes, these stripes will be reddish to purple. The lesions have well defined margins are delimited by the leaf veins. The disease may first be noticed on the lower leaves; severely affected leaves may be shed prematurely. Infection has been associated with smaller seed size. Early plant death can result when severe disease occurs prior to flowering. Infection does not cause malformation of vegetative or floral tissues. Under conditions of high moisture, sporangial growth appears as a grayish-white woolly growth on both the upper and lower surfaces of lesions. Primary inoculum comes from oospores overwintering in soil or plant debris or from mycelium in infected seed. The corn seed may be contaminated from leaf debris on its surface or can have oospores as well as mycelium within the embryo. The former route is more likely to initiate new infections than the latter. Oospores in air-dried leaf tissue can remain viable for 3 to 5 years. Oospores usually undergo indirect germination, producing sporangiophores that bear sporangia which may contain four to eight zoospores. Sporangia are dispersed by wind and water, or from physical contact with an infected plant. Moisture is essential for infection. Twelve hours of leaf wetness were required for infection via zoospores. Use resistant varieties.
Do not rotate or simultaneously cultivate maize with alternate hosts of downy mildew. Remove infected crop debris if possible. Treat seed or crop with systemic fungicide. Plant when soil temperature is below 20°C which is unfavorable for oospore germination. Ensure seed has low moisture content (below 9%) before planting. Control weeds and reduce crop density to reduce moisture levels by increasing aeration.