The effect of fungicides on survival of the crayfish plague fungus, Aphanomyces astaci, Oomycetes, growing on fish scales.

Zoospores from the crayfish plague fungus Aphanomyces astaci could germinate and grow in vitro on scales of Salmo salar. The fungicidal activity of different compounds was tested against the fungus growing on fish scales as well as in lake water. Malachite green was the most effective fungicide, but scale mucus protected the fungus from this compound to some extent. For complete inhibition of spore germination in scale mucus 2 mg1-1 was required compared to 1 1 mg1-1 in lake water.

Blue light inhibits mycotoxin production and increases total lipids and pigmentation in Alternaria alternata.

Light inhibits production of the mycotoxins alternariol and alternariol monomethyl ether, both polyketids produced by Alternaria alternata. This effect seems to be general because seven isolates of A. alternata with different alternariol- and alternariol monomethyl ether-producing abilities all respond to continuous light with reduced levels of alternariol and alternariol monomethyl ether when the mycotoxins were calculated on a microgram-per-milligram (dry weight) basis. Blue light inhibited alternariol and alternariol monomethyl ether production 69 and 77%, respectively. Red light gave no reduction of toxin levels. Total lipids were increased 25% when mycelium was grown in blue light as compared with red light or darkness. In white or blue light, but not in red light or darkness, a red-brown pigment accumulated by the mycelium.

Activation of serum prophenoloxidase in arthropod immunity. The specificity of cell wall glucan activation and activation by purified fungal glycoproteins of crayfish phenoloxidase.

This study shows that the activation of crayfish serum prophenoloxidase by carbohydrates was specific for beta-1,3-glucans. Fractionation of the beta-1,3-glucan laminaran into laminaran M and laminaran G showed that both activated the proenzyme, but the G-chain had somewhat higher affinity for the proenzyme. Methylation analysis of these two fractions revealed that there were no 1,6-linkages present. Laminaripentaose, a linear pentasaccharide composed of (1 leads to 3)-linked beta-D-glucopyranosyl residues was also active but had a lower affinity for the proenzyme than laminaran G. Laminaran completely inhibited the activation of prophenoloxidase by the pentaose. In the concentrations tested, laminaran was not inhibitory to the phenoloxidase activity. Purified extracellular glycoproteins of the parasitic fungus Aphanomyces astaci also strongly activated crayfish serum prophenoloxidase. Only high molecular weight glycoproteins were effective. Exo-beta-1,3-glucanase treatment decreased the activating capacity, suggesting that at least part of the glycoproteins consisted of beta-1,3-glucans. The significance of these results in the defence against parasitic fungi in crayfish is discussed.

Light inhibits the production of alternariol and alternariol monomethyl ether in Alternaria alternata.

Alternaria alternata (Fr.) Keissler, grown in drop culture, produced alternariol and alternariol monomethyl ether in late growth phase. Production was almost completely inhibited when the fungal cultures were exposed to white light (180 W/m2), although mycelial dry weight was not significantly affected. The fungus was most sensitive to light during the exponential growth phase. Twelve hours of light exposure was sufficient to decrease significantly the production of the secondary metabolites. In light the fungus produced a red-brown pigment of unknown nature.

Cyst and germ tube wall structure in Aphanomyces astaci, Oomycetes.

Aphanomyces astaci secondary cyst walls and walls of germinating spores were prepared by alkaline hydrolysis, autolysis, sonication, and enzymic degradation and were examined by shadow-casting and negative-staining techniques. The cyst wall consists of randomly oriented fibrils, about 3 nm in diameter. The fibrils are embedded in, or covered by, amorphous beta-1,3-glucans which can easily be removed by alkaline hydrolysis. The germ tube wall surface has the same structure, but the amorphous layer is less easily removed.

Defence reactions in and susceptibility of Australian and New Guinean freshwater crayfish to European-crayfish-plague fungus.

The deposition of melanin of the surface of the cell wall of the crayfish-plague fungus, Aphanomyces astaci, in infected cuticles of Australian crayfish seemed to be correlated with some degree of resistance to infection. The most obvious reactions towards the fungal structures present in the blood in vivo were their encapsulation by blood cells, many which then disintegrated. This was followed by melanization of the fungal surface. Crayfish blood inhibited completely hyphal growth in vitro and the disintegrating blood cells were apparently responsible for this effect as well as for the melanization. Phagocytosis of fungal structures was not observed. Melanization in vivo, and in vitro, due to a polyphenoloxidase released by the blood cells, was stimulated by the presence of the fungal cell wall surface. This seems to be a typical reaction of the crayfish in general. It was known earlier that European and Japanese freshwater crayfish are very susceptible to attack by the cray-fish-plague fungus but that North American cray-fish have a higher, although not complete, resistance and may be the natural hosts for the parasite. Nine species from different parts of Australia and from New Guinea were tested in aquaria and were all found to be susceptible, often very susceptible. Their susceptibility to infection was very similar to that of the European species, and the results support strongly the hypothesis that the parasite originated in North America and not in the Australian region or elsewhere. It would appear that man has not yet introduced the disease to the Australian region. Should this, however, occur, it will probably prove disastrous to the Australian and New Guinean freshwater crayfish fauna.

Mycoplasma-like structures in the aquatic fungus Aphanomyces astaci.

The hyphae of a nonzoosporulating strain of Aphanomyces astaci contain numerous structures which most closely resemble published micrographs of certain mycoplasmas. Two normal strains of the same species do not contain these mycoplasma-like bodies.

Cytochromes of aquatic fungi.

The cytochrome systems of two classes of aquatic fungi, the Oomycetes and Chytridiomycetes, were studied by means of reduced-minus-oxidized difference spectra at room and at low temperature. At room temperature, all of these fungi have a c-type cytochrome with an absorption maximum at 551 mmu and a b-type cytochrome at 564 mmu. The Oomycetes have a-type cytochromes at 605 mmu, and the Chytridiomycetes have a-type cytochromes at 606 mmu (Blastocladiales) or at 609 mmu (Monoblepharidales). Additional b-type cytochromes are found at 557 mmu in the Oomycetes and at approximately 560 mmu in the Chytridiomycetes. The data obtained from spectra at low temperature are consistent with these conclusions. Thus, the difference spectra reveal variation between the cytochrome systems of these two classes of aquatic fungi.