Important taxonomic differences betweenVerticillillnl al bo-atrllnl, V. dahliae, and V. tricorplls occur in the morphology of the conidiophores and the conidia. These differences are additional to the morphological differences in the resting mycelium, and justify the recognition of different pathogenic species of Verticillillnl, causing wilt diseases in a wide range of hosts.Verticilliunl al bo-atrunl on natural host material typically has larger conidiophores with a dark swollen base; larger conidia. mainly 3.5-8 X 2-3/1. occasionally one septate; dark thickened resting mycelium and dark swollen hyphae; hyphal "knots" but no microsclerotia; a lower temperature range for growth on media and pathogenicity; a more restricted host range; a greater degree of pathogenicity than V. dahliae at 20 c e towards many hosts (including tomato, cucumber, and strawberry).Verticillillnl dahliae on natural host m'lterial typically has fewer. smaller. completely hyaline conidiophores. smaller conidia. mainly 3-6 X 1.5-2JL; abundant black microsclerotia; a higher temperature range for growth on media and pathogenicity; a very extensive host range; a lower pathogenicity than V. albo-atrunl on many hosts.Verticillillnl tricorplIs on inoculated tomato stems typically produces hyaline conidiophores with a swollen light tan base; large conidia, mainly 6-\5 X 2.5-4/1; dark thickened hyphae; dark torulose hyphae; black microsclerotia. and has a high temperature preference. and is usually a weak pathogen.As in other countries, there are host and geographic distribution differences for the two Verticillillnl species found in New Zealand. Verticillillnl albo-atrunl has been found causing economic losses only on potatoes, tomatoes. lucerne. and strawberries in the southern (colder) districts of New Zealand. Verticillillnl dahliae. however, causes economic losses on tomatoes, tobacco, eggplant. strawberries. gooseberries. apricots, and roses. particularly in the northern (warmer) districts and in hot summers. V. tricorplIs has not yet been isolated in New Zealand. but was described from a culture first received in 1949 from Dr S. Wilhelm. University of California. Cultures typical of these three species were examined by Dr r. Isaac. He stated that they corresponded with his interpretation of these three species.N.Z. J. agric. Downloaded by [171.25.193.25] at 15:

Ten years after clear-cutting forest stands in Virginia and West Virginia, competing hardwoods around American chestnut trees were either periodically cut (managed) or not cut (control or unmanaged). Blight epidemics occurred in all clear-cut plots prior to their establishment in 1984. For the control plots, the number of chestnut sprout clusters (group of stems sprouting from a single stump) with blighted live stems had decreased by 1989, but between 1985 and 1988 there was a general increase in blighted chestnut stems for the managed clearcut plots. The number of apparent superficial cankers increased in two of three managed plots between 1986 and 1989. Survival of chestnut clusters was high in managed plots between 1985 and 1989, but decreased in one control plot to zero by 1988. Mean diameters at breast height of measurable chestnut stems (> 0.8 cm) declined or remained constant in control plots, but generally increased in managed plots after initial declines. Similar but greater responses were observed in a plantation of American chestnut. Unmanaged clearcuts with a high survival of chestnut clusters had low stand basal areas for competing hardwoods, and vice versa. A mesic site favored American chestnut growth and apparent superficial cankers in the absence of competing hardwoods. Key words: chestnut blight, hypovirulence, biocontrol, forest competition.

1. Barley yellow dwarf virus (BYDV) is the most important disease affecting cereal crops in New Zealand. 2. Losses from BYDV in the total wheat crop have been up to 25 per cent in recent years. 3. Effective control of BYDV in autumn-and early winter-sown wheat in New Zealand has been achieved by 1 application of a good systemic organo-phosphate spray (Metasystox i, Rogor 40, Ekatin, or Phosdrin) at any time between late winter and late spring. 4. These spray materials applied at rates between 6 oz and 16 oz of commercial material in 9 to 15 gal of water per acre by tractoror helicopter-mounted booms have given excellent control of BYDV and the aphid vector Rhopalosiphum p adi, and increases in yield of 5 to 24 bushels of wheat per acre, even when very low or very high aphid numbers were present in the field at the time of application. 5. It is thought that the full significance of late secondary spread of BYDV in reducing yields of relatively tolerant cereals, particularly wheat and barley, has not been recognised in many countries throughout the world.

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The development of Eryszphe graminis f. tritici Em. Marchal, from spore germination to spore formation, occupied 6 days and ascospore discharge in the field covered a period of I month. On immune hosts development of the fungus generally ceased at the point of invasion of cell cytoplasm. The features of host plant resistance have been discussed.Lithium chloride soil applications were made as 0.8% solution in glasshouse seedling plant experiments and at 56 lb./acre rate in a factorial field trial. In the former, mildew was reduced from I I % leaf area infected to I % , but under field conditions only from 1-29 to 1.04% over two sampling periods. The limiting effects of lithium on mildew began to decline 18 weeks after soil treatment.Race 4 of E . gramirris f. tritici (Canadian typing) was present in all, and Race 3 in about half the New Zealand wheat localities during 1948-9. The infection types of Races 3 and 4 on 180 wheat varieties and breeding selections have been recorded, and the basis for a mildew resistance plant-breeding programme is discussed, including consideration of adult plant resistance.