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Two cultivars of Cymbidium orchid were mechanically inoculated with Odontoglossum ring- spot virus (ORSV) and Cymbidium mosaic virus (CyMV), individually and in combination. ORSV was found to have an infection rate of 70% (as determined by ELISA), but seldom induced easily discernable leaf symptoms. CyMV had an infection rate of only 20%, but infected plants invariably produced a pronounced leaf mosaic either with or without necrotic streaks. Both viruses were found to reduce plant growth, the effects of CyMV being more severe than those of ORSV. [ABSTRACT FROM AUTHOR]

Trichoderma cultures were isolated locally from soils and screened on potato-dextrose agar against Rhizoctonia solani. Several isolates of T. harzianum reduced the growth and build up of populations of R. solani and to a lesser extent of Fusarium solani in sterilized soil. In field experiments, biological control of R. soiani and F. solani infections in tobacco transplants was achieved by adding T. harzianum (T77) to methyl-bromide-fumigated seedbeds before seed was sown. There was also increased growth and leaf yield that was not directly correlated with disease control. Triadimenol fungicide, integrated with the Trichoderma treatment, enhanced disease control. [ABSTRACT FROM AUTHOR]

Triadimenol residues in sandy and clay soils, following applications of up lo 1 g/m² in previous seasons to tobacco seedlings 2 days before they were transplanted, had little effect on seedling emergence and growth. Residues were beneficial to transplants; they improved growth and controlled stem and root infection caused by Rhizoctonia solani and Fusarium solani. During the first 4 weeks after transplanting, stem infection was negatively correlated with loge of the pre-sowing rate of triadimenoi and with loge of the soil concentration in seedbeds at pulling. Plant growth increased linearly according to the equation 1/Y=a + b/(x - c ), where Y = plant dry mass (g/plant), x = triadimenol pre-sowing (mg/m²) and c = - 1.1, determined iteratively, r²- = 0.94; P < 0.001. Triadimenol applications to bare soil increased the soil concentration by 1 μg/g per 50 mg/m² applied. [ABSTRACT FROM AUTHOR]

The population dynamics of two strains of Pseudomonas syringae pv. tabaci that cause the foliar spot diseases of tobacco known as wildfire (WF) and angular leaf spot (ALS) were significantly different in leaves of a susceptible tobacco cultivar. After 48 h the population of WF strains changed abruptly from exponential growth to stationary phase, whereas there was a more gradual transition by the ALS strains. This resulted in smaller stationary-phase populations for WF than for ALS. The decline of the WF population started soon after the stationary phase was reached, whereas the ALS stationary phase was relatively stable. The strains reacted differently to climatic factors. On plants inoculated and exposed to the weather at weekly intervals, WF lesions grew faster when vapour pressure deficits, maximum temperatures and radiation were low. Multiple regression analysis showed that over 70% of the variation in WF infection was accounted for by variation in VPD, radiation and mean temperature. Variation in ALS infection was less easily explained in relation to the weather variables measured. Plant age and stalk position had a differential effect on the pathogenicity of the two strains. Lower leaves of young plants (42 days) were most susceptible to WF and top leaves of older plants (84 days) were most susceptible to angular leaf spot. Differences in pathogenicity and occurrence in the field suggest that pathovar status should be considered for the ALS pathogen of tobacco. [ABSTRACT FROM AUTHOR]

A blackening confined to the cuticle and/or outer epidermal cell walls of mature leaves of Dioscorea alata was found to be associated with the appressoria of the fungus Colletotrichum gloeosporioides on plants subjected to high levels of inoculum in the field and to dense spore suspensions in controlled tests. The blackening occurred with little evidence of fungal penetration into the host cells. Preliminary analyses of the nature of the blackening detected melanin-like compounds and suggest the involvement of phenolic compounds and water-soluble chemicals, possibly enzymes, in the blackening process. [ABSTRACT FROM AUTHOR]

A detailed account of symptoms produced by Colletotrichum gloeosporioides on susceptible and resistant cultivars of yam (Dioscorea alata) in Solomon Islands is given. A superficial blackening of upper leaf surfaces of some cultivars is unusual and worthy of further study. In pathogenicity tests with isolates of C gloeosporioides from various sources, those that were virulent on D. alata were all from D. alata except one from D. esculenta and one from Cucumis sativus but only nine of 30 isolates from D. alata were virulent on this host. Since all four isolates of the yam pathogen tested on D. nummularia and D. trifida were virulent, it is inappropriate to retain the epiphet f. sp. alatae proposed by earlier authors. Considerable variation in cultural morphology existed among virulent isolates, the only consistent character being production of dark pigment, although this was also produced by some saprophytic isolates. Most reisolations of C. gloeosporioides from lesions resulting from inoculation yielded isolates identical with the original inoculum but also some reisolates had a different morphology. [ABSTRACT FROM AUTHOR]

In an experiment in an unheated glasshouse in which tomatoes were picked for about four months, yields from annually cropped soils decreased from 4.48 to 2.71 kg/plant when soil remained untreated for a second year, the decrease being associated with increasing attack by Pyrenochaeta licopersici. Partial sterilization in 1966 with chloropicrin or methyl bromide decreased brown root rot (BRR) in previously untreated soil from 60 per cent to 11 and 12 per cent and increased yields from 2.71 to 4.96 and 4.30 kg/plant respectively. Yields were unaffected when soils, fumigated with methyl bromide in 1965. were partially sterilized with chloropicrin or methyl bromide in 1966. When cylinders of soil 0.23 m diameter and 0.23 m deep, at planting sites within untreated plots were replaced with heat sterilized soil, BRR decreased from 60 to 11 per cent and yields increased from 2.71 to 3.70 kg plant. This effect was attributed to delayed infection of older roots within the sterilized soil, because the slow growing P. lycopersici had to extend further from surrounding infestations. The largest yields (c. 5.85 kg/plant) were obtained, irrespective of soil treatment, from cv. Potentate tomatoes grafted to KN rootstocks resisting P. lycopersici. [ABSTRACT FROM AUTHOR]