Oomycetes

There is abundant evidence for the presence of pheromones regulating sexual reproduction by Oomycetes, the classical example being the steroid pheromones produced by Achlya spp. (reviewed by Barksdale 1969 Horgen 1977, 1981 McMorris 1989). Sexual reproduction (oosporogenesis) is initiated by secretion of a steroid pheromone, antheridiol, and a variety of less biologically active 

In addition to Achlya, there is some evidence supporting the existence of the production of sexual pheromones in other Oomycetes, especially among the plant pathogenic members of the Pythiaceae (Sherwood 1966 Hendrix 1970 Pratt et al. 1972 Brasier 1975 Ko 1978 Zentmeyer 1979 Shen et al. 1983). Several other methods have been developed for indirectly following the production of these compounds. One involves the use of polycarbonate membranes, which are 

A second approach used cheese cloth impregnated with various biological extracts (Zaki et al. 1983). Extracts (or commercially available standards) are dissolved in an appropriate volatile solvent and applied to sterilized cheese cloth. The solvent is allowed to evaporate, and agar blocks with the experimental organism placed between two sheets of the cloth. The blocks are incubated in sterile distilled water in a petri plate for 2-24 days with periodic microscopic examination for sexual or asexual propagules. 

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Onyxide 200 Oocysts Oolites Oolong tea Oomycetes Oospora destructor Oospora sp. 02-oxidoreductase OP-2507 [101758-79-6] Opacifiers Opacifying agent... 

Phycomycetes subclass oomycetes P/asmopara viticola downy mildew of grape... 

ROS production by an extracellular agar oligosaccharide oxidase Appressoria formation in the specific pathogenic oomycete Pythium porpyrae... 

Unlike fungal saponinases, little is known about hydrolytic enzymes secreted by phytopathogenic oomycetes or their role in pathogenicity [75, 76]. Although innate resistance is believed to be the main mechanism by which oomycetes avoid the toxicity of saponins (see above), some oomycetes have been found to produce saponin hydrolases [77], as well as other glycosyl hydrolases [76]. 

Brunner F et al (2002) A 6-glucosidase/xylosidase from the phytopathogenic oomycete,... 

Wood SG, Gottheb D (1978) Evidence from mycelial studies for differences in the sterol biosynthetic pathway of Rhizoctonia solani and Phytophthora cinnamomi. Biochem J 170 343 Nes WD et al (1986) A comparison of cycloartenol and lanosterol biosynthesis and metabolism by the Oomycetes. Expeiientia 42 556... 

Mikes V (1997) The fungal ehcitor cryptogein is a sterol carrier protein. FEES Lett 416 190 Ponchet M et al (1999) Are ehcitins cryptograms in plant-Oomycete communications Cell Mol Life Sci 56 1020... 

Novel cyclohexenones, acremines A-F (109-114), were isolated from a mycoparasitic Acremonium sp. from the oomycete pathogen Plasmopara viticola on Vitis vinifera cv. Regina blanca. Compounds 109-112 inhibited germination of the pathogen, and the most activity was shown by acremine C (111). This work may lead to a mechanism for the mycocidal activity of mycoparasitic fungi. 

Schardl CL, Craven KD, Interspecific hybridization in plant-associated fungi and oomycetes A review, MolEcol 12 2861—2873, 2003. 

Jacobson DJ et al. Persistent, systemic, asymptomatic infections oJAlbugo Candida, an oomycete parasite, detected in three wild crucifer species. Can J Bot 76 739-750, 1998. 

The Oomycetes, a class within the Phycomycetes, is unusual and regarded by many as being distinct from the true fungi. The taxonomic... 

Copper, as copper sulfate, was first used to control Tilletia caries in wheat but its main use, as Bordeaux mixture, is in the control of Oomycetes in a wide spectrum of crops such as potato, tomato and vines or in combination with systemics such as cymoxanil. 

Benzimidazoles are highly selective. Oomycete fungi are insensitive to the benzimidazoles, as are higher plants. The reason for the distinct differences in sensitivity is unknown but probably depends on single structural differences of the microtubule binding site. Resistance of this type, if stable, spreads rapidly and results in catastrophic disease control... 

Fosetyl. Fosetyl (aluminium ethyl phosphate Figure 4.32) is a specific downy mildewicide (Oomycetes) used to control Plasmopara viticola in vines and Phytophthora blights in various fruit and nut crops. 

Prothiocarb, Propamocarb. Prothiocarb (superseded compound) and its oxygen analogue propamocarb (Figure 4.33) are selectively active against Oomycetes. Propamocarb, as its hydrochloride, is xylem mobile with uses as a seed treatment, drench, soil incorporation, dip or foliar spray in the control of diseases caused by Oomycetes on ornamentals, tobacco, fruit and potatoes. 

Cymoxanil. Cymoxanil (Figure 4.34) is a systemic urea with protectant and curative activity against Oomycetes, particularly Plasmo-para viticola on vine and Phytophthora infestans (potato late blight). 

Dimethomorph. Dimethomorph (Figure 4.36) is effective against Oomycetes except Pythium spp. The compound, which is a cinnamic acid derivative, operates as a protectant but also has some curative activity that can be modified by means of formulation. Only the Z isomer is active but because of the rapid interconversion of isomers in the light, there is no practical advantage in its specific synthesis. 

Recent phylogenetic analysis showed that ferredoxins most similar to I vaginalis ferredoxins 1-3 are found in Tritrichomonas foetus and in the oomycete Phytophthora soaje. Proteins from G. intestinalis, red algae of genus Cyanidioschyzon, and also from the hyperthermophilic eubacterium Aquifex are also similar to I vaginalis ferredoxins (our unpublished data). 

This primary mode of action of tomatine, that involves the formation of complexes with membrane sterols is similar to that described for polyene antibiotics [2, 4], and results in pore formation and loss of membrane integrity. This mode of action is supported by the reduced activity of tomatine on sterol-free bacteria and Oomycete fungi such as Pythium and Phytophthora [15, 28], and the strongly reduced toxicity of hydrolysis products of the glycoalkaloid which fail to bind sterols [57]. 

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