No description available.

The morphology, physiology and pathology of two isolates are described. The first in Part I is called 'India' having come from a banana plant in Madras, and E is compared with V.theobromae isolates and V.dahliae. The second isolate described in Part II is called 4655. It was thought to be a V.albo-atrum with peculiar brown spores measuring 15P in diameter - these were discovered to be Acanthamoebae polyphaga cysts. This isolate is investigated with and without its contaminant and compared with V.albo-atrum. 'India' and the V.theobromae isolates were all seen to produce microsclerotia, resting hyphae and colourless chlamydospores. Large hyphal cells were also seen. 'Melanised' excrescences on the resting hyphae and conidiophores of 'India' were an outstanding feature on Dox's agar, and all the V.theobromae isolates produced them to a lesser extent. 'India's' optimum temperature for growth was 25°C but it and V.th.8 still grew at 35° C. Its optimum pit was c7.3. 'India' was the only isolate to change Amended Dox's Agar from more acid to alkaline. Plate coverage was maximal on 5% maltose, but all additions of maltose gave the best results. Additions of sucrose, maltose and glycerine were only slightly inferior in their results. With different nitrogen sources plate coverage was best on media containing 0.1% asparagine, 0.1% peptone and 1% sodium nitrate plus 0.1% calcium carbonate. Little growth took place with 1% sodium nitrate plus 1% calcium carbonate and in general poor results were obtained with ammonium nitrate. 'India' was pathogenic to tomato and sunflower plants being reisolated from both hosts. It was decided from all the results obtained that 'India' was an isolate of V.theobromae which was confirmed by the C.M.I.4E5 was morphologically very similar to V.albo-atrum except that it produced more colourless chlamydospores and more excrescences on resting hyphae and conidiophores. It also produced a small collarette when a conidium was abstricted. Large hyphal cells were again seen, and in all cultures narrow colourless hyphae with 'melanin' spots inside occurred. In cultures with altered nitrogen content conidia with 'melanin' spots were found. Both physiologically and pathologically 4E5 behaved very much like V.albo-atrum_ except that it was a more virulent fungus so is accepted as a V.a-a isolate. Appendix 1 is included in order to suggest a means of eradicating mites. Appendix 2 deals with the extremely common presence of aerial resting structures - seen on every isolate examined.

The host-pathogen interaction of lucerne (Medicago sativa L.) with isolates of the vascular wilt fungus, Verticillium, was investigated. Pathogenicity trials showed that only isolates of Verticillium from lucerne were pathogenic to this host. Lucerne cultivars resistant to European isolates of the pathogen were not resistant to the more virulent American isolates of the fungus. The production of phytoalexins by the host in response to isolates of Verticillium was investigated. The specialised pathogenicity of the lucerne isolates was correlated with their induction of low levels of phytoalexins in the host while non-pathogenic isolates of Verticillium induced high phytoalexin levels in lucerne. Several factors e.g. incubation period and age of leaf material affected the levels of phytoalexin which accumulate in the diffusates of lucerne leaflets. An investigation of the induction mechanism for phytoalexins in lucerne revealed that culture filtrates and spore free filtrates did not provide sufficient stimulus for phytoalexin induction. Of treatments used, only viable Verticillium spores and copper chloride solutions induced phytoalexin production. Penetration of the epidermal layer of lucerne leaves by a non-pathogenic isolate of Verticillium resulted in the rapid necrosis of epidermal cells and an associated accumulation of phytoalexins in both leaf diffusate and leaf tissue. It is suggested that the reaction of the host to the non-pathogenic isolate represents a hypersensitive response i.e. rapid cell necrosis with associated accumulation of phytoalexins. In contrast, penetration by the pathogen took several days. Although phytoalexins were eventually produced, their production did not limit the growth of the pathogen and the whole leaf became necrotic and ramified with mycelium.

The reported pathogenicity of V. albo-atrum and V. dahliae to potatoes and antirrhinums was confirmed. Pectic enzyme secretion by V. albo-atrum and V. dahliae was studied in vitro, in cultures grown on pectin, sodium polypectate, and extract cell wall material from both potato and antirrhinum respectively. The enzymes were produced sequentially over two to twelve days. Endopolygalacturonase (endo PG) was the first formed enzyme in this sequence, followed by exopolygalacturonase (exo PG), and pectin transeliminase (PTE). V. alboatrum generally exhibited higher pectic enzyme activity than V. dahliae. The production of these enzymes was subjected to varying degrees of induction and repression when a high or low concentration of D-galacturonic acid or D-glucose was present in the medium. Properties of enzymes were investigated in terms of pH optima, fungal growth, maceration and cell death of potato tissue, mechanism of substrate degradation, purification and molecular weight determination. Studies on plants infected by V. albo-atrum and V. dahliae suggested an involvement of endo PG in the production of wilt symptoms. These were: a) detection of endo PG but not exo PG or PTE in infected plants and in infected cuttings; b) reproduction of symptoms in potato and antirrhinum cuttings with partially purified endo PG. An extract from the cell walls of both potatoes and antirrhinums was found to inhibit the endo PG secreted by both V. alboatrum and V. dahliae. There was no corresponding inhibition of the exo PG or MI enzymes. The significance of the formation of "blisters" in vascular vessels in response to infection by V. albo-atrum and V. dahliae is discussed in relation to the vascular discolouration. The formation of thin outgrowths from hyphae to vessel walls in the xylem of potato stems and the occurrence of "blister" are highly related. A solid medium on which the production of extracellular pectic enzymes by several isolates of V. albo-atrum and V dahliae, as well as V. tricorpus, V. nubilum and V. nigrescens can be detected is described.

The production of the isoflavonoid phytoalexins medicarpin, sativan and vestitol by lucerne (<i>Medicago sativa</i> L) in response to substances, named elicitors, released into culture fluids of the plant-pathogenic fungus <i>Verticillium albo-atrum</i> was investigated. Methods for establishing the production of phytoalexins were studied and developed to use for screening substances from culture filtrates for elicitor activity. A method of reverse-phase High Performance Liquid Chromatography was developed in order to obtain quantitative measurements of elicitor activity. The presence of elicitors in extracts of the mycelium of the fungus was also investigated. Attempts were made to isolate and purify the component(s) from culture filtrates and mycelial extracts of <i>Verticillium</i> responsible for elicitor activity. Activity was not removed from culture filtrates by dialysis. Studies were made initially using Sephadex G-200 gel chromatography and anion exchange chromatography. Subsequently, the culture filtrate elicitor was found not to be precipitated in 80% v/v ethanol while inactive components were. Biogel P-10 chromatography produced an apparently homogeneous fraction on which preliminary chemical studies were performed. A molecular weight of 3000-4000 was indicated for the material. This fraction proved capable of further resolution. Cation and anion exchange chromatography was used for this purpose. The most active fraction obtained was retained by the anion exchanger but not a cation exchanger. This could be resolved by analytical scale reverse-phase liquid chromatography into multiple components. Colorimetric assays showed the material to be predominately peptide although a small amount of carbohydrate was present. The most active material from mycelial extract was not retained by either cation or anion exchangers. Colorimetric assays showed it to be predominately carbohydrate although protein was also detectable.

No description available.

No description available.

Investigations on the infection of some monocotyledonous plants by Verticillium spp. showed that they were susceptible only to V. dahliae. This was shown by the colonization of the root surfaces and superficial tissues, although V. dahliae was usually present in the microsclerotial form and the viability of these propagules was confirmed. Investigations involving studies of the effect of different crop cover on the soil population of V. dahlias showed that both susceptible and resistant plants increased the number of propagules in the soil. The penetration of wheat roots by V. dahlias in culture was studied. Here light and electron microscopy were used to study the infection process. The light microscope showed that the plant was able to resist vascular penetration by the fungus. This resistance to penetration appeared to be due to the formation of barriers such as lignification and lignituber formation.in the cell walls. Varying the pH of the medium and growing the seedlings and fungus at different temperatures had no apparent effect on the extent of penetration. Resistance appeared to be a feature of living cells presenting an active response to infection. A comparison of spore germination on susceptible and resistant roots indicated that there was no inhibitory substance/s produced by the resistant plant. Transmission electron microscopy presented convinving evidence that resistance to penetration was due to the formation of deposits on the innerface of host cell walls (referred to as lignitubers). In electron micrographs these lignitubers appeared as membranous pads composed of alternating electron dense and electron lucent material. They were apparently formed by the host cytoplasm as a result of the aggregation and fusion of protoplasmic vesicles with the host plasmalemma. The host cell wall produced "covering material" which appeared to be a modification of the wall. This material formed around any hyphae that were in contact with the wall and possibly served to suppress infection. Histochemical tests of the "covering material" showed that it was composed of cellulose. The penetration of wheat root by a virulent fungus, viz. Fusarium culmorum was also studied by light and electron microscopy and it was found that this fungus penetrated the wheat root with little or no host reaction.

No description available.

No description available.