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Botrytis cinerea and Sclerotinia sclerotiorum are the two most serious pathogens on kiwifruit in New Zealand. Because of the pesticide regulations in some of the countries to which New Zealand exports fruit, total protection from Botrytis stem end rot with current dicarboximide fungicides is not possible. The aim of this thesis was to investigate biological control measures for Botrytis stem end rot and Sclerotinia diseases of kiwifruit. More than 1000 microorganisms, isolated from the leaves and flowers of kiwifruit during spring and autumn, and selected from BCAs reported to be effective against B. cinerea and./or S. sclerotiorum, were tested in vitro for their antagonistic ability against B. cinerea and S. sclerotiorum. Successful antagonists were those that, in dual culture on agar plates, produced a zone of inhibition, an area of browning of the pathogens, or grew rapidly over the pathogens and inhibited their growth. The fifty most promising isolates from the initial screen were tested on fruit for their ability to reduce Botrytis and Sclerotinia fruit rots. Mature kiwifruit were artificallv wounded and dual inoculated with a spore suspension of one of the fifty test organisms and either a conidial suspension of B. cinerea or a mycelial suspension of S. sclerotiorum. Following 8-12 weeks incubation in a cool store, fruit were assessed for Botrytis or Sclerotinia induced rot. Isolates of Bacillus spp., Epicoccum purpurascens, Pseudomonas sp. and Trichoderma. spp. reduced, Botrytis fruit rot from 92% (inoculated control) to 0%.Isolates of Alternaria spp., pestalotia sp. and a non-sporulating isolate also reduced the number of fruit rotting to some extent. Similarly, isolates of Bacillus spp., E purpurascens and Trichoderma spp. reduce d Sclerotinia fruit rot from 100% (inoculated control) to 0%. Isolates of Alternaria spp., Myrothecium verrucaria and Pestalotia sp. were also successful at reducing the level of Sclerotinia fruit rot. It was considered undesirable if potential biological control agents (BCAs) were able to colonize kiwifruit that were to be marketed for human consumption. In order to determine if microorganisms, shown to be effective in preventing Botrytis or Sclerotinia fruit rot, were capable of themselves colonizing fruit, isolations were made from fruit dual inoculated with B. cinerea, S. sclerotiorum and/or one of several BCAs. Strains of the BCAs Bacillus spp., Pseudomonas sp. and E. purpurascens were not found to be saprophytic on fruit. Isolates of Alternaria sp., Bacillus sp., E purpurascens, pestalotia sp., Pseudomonas sp. and T. harzianum significantly inhibited germination and germ tube elongation of B. cinerea conidia in vitro in a nutrient solution, over a 24 h period. For example, the presence of Alternaria alternata A6 spores in a nutrient solution reduced germination of B. cinerea conidia from 100% to 20%. The presence of E purpurascens A77 spores inhibited B. cinerea conidial germ tube elongation from >840 pm (in control conidia) to 27 µm. The presence of any one of the BCAs tested prevented germination of B. cinerea conidia in a non-nutrient water solution, in comparision to germination of up to 86% in controls. A spore or cell suspension of each of the isolates Bacillus sp.M60, E. purpurascens A77 and T. harzianum C65 were spray inoculated onto kiwifruit blossoms produced in vivo in the glasshouse, immediately prior to inoculation of the blossoms with a condial suspension of B. cinerea. Application of the BCAs were completely effective in preventing colonization of blossoms by B- cinerea conidia. The effectiveness of each of the isolates E. purpurascens A77,T. harzianum C65 and either Bacillus sp.M60 or M53 to reduce the viability of sclerotia of B. cinerea and S. sclerotiorum was tested in soil punnets. A spore or cell suspension of each respective BCA was applied to the surface of replicated punnets that were seeded with either B. cinerea or S. sclerotiorum. Following 8 weeks incubation, punnets were harvested and viability of sclerotia assessed. T. harzianum C65 and Bacillus sp. M60 significantly reduced the viability of B. cinerea sclerotia from 8 sclerotia/punnet (control) to 4 sclerotia/punnet. T. harzianum C65 and E. purpurascens A77 caused a significant reduction in apothecia production of S. sclerotiorum, from 2.7 apothecia/punnet (control) to 0.7 apothecia/punnet. Bacillus sp.M8 and E purpurascens A77 were tested for their ability to reduce Botrytis stem end rot and Sclerotinia field rot in a kiwifruit orchard. The isolates tested did not successfully reduce either disease. Possible explanations for this are discussed. In order to monitor the survival of particular isolates of BCAs in the field, a technique was developed to distinguish between individual strains of a BCA species. The polymerase chain reaction (PCR) was utilized to identify DNA polymorphisms within the genome of T. harzianum C65, in comparison with other strains of Trichoderma spp.. A sequence of polymorphic DNA was cloned, sequenced and used as a hybridization probe in southern blotting to enable T. harzianum c65 to be distinguished from other strains of Trichoderma spp.. From the results obtained in this study, it was considered that Bacillus M60, E purpurascens 477 and Pseudomonas M30 were the best isolates for the biological control of Botrytis stem end rot on kiwifruit. Further work to enable application of these isolates as postharvest BCAs is discussed. Of the isolates tested in this study, T. harzianum C65 was considered the best isolate for use against Sclerotinia diseases on kiwifruit. Methods of selecting more effective BCAs against S. sclerotiorum are discussed.

1. When purified turnip ye1low mosaic virus was inoculated mechanically on to Chinese cabbage leaves, using known numbers of virus particles in 0.1 to 1.0 µ1 volumes of inoculum, as few as 10 to 30 particles were required to produce a single local lesion.2. Inoculation of a cotyledon leaf of Chinese cabbage seedlings with turnip yellow mosaic virus produced a rapid transient inhibition in the rate of leaf initiation, so that infected plants developed 0.5 to 1.0 leaf less than healthy plants. 3. The factor that initiated the inhibitory response a t the apical. meristem began moving out of the inoculated cotyledon within 1to 6 hours after inoculation, thus preceding the movement out of the inoculated cotyledon of infectious virus or RNA which was not detectable until about day 5. 4. The transient inhibition of leaf initiation occurred following inoculation with any one of three unrelated viruses, or with infectious turnip ye1low mosaic virus RNA. 5. A factor eluted in an active form from the cut petioles of inoculated 1eaves. 6. It is necessary to inoculate with infectious virus or RNA to initiate the production of the inhibitory factor. 7. No differences were seen in the magnitude or timing of the reduced rate of 1eaf initiation, when the concentration of turnip yellow mosaic virus in the inoculum was varied between 1 µ g/ml and 100 µg/ml. 8. Inoculation of the cotyledons of Chinese cabbage seed1ings with turnip ye1low mosaic virus caused a marked disturbance in the mitotic index a t the apical meristem between 6 and 48 hours. 9. A reduction in the accumulation of starch in the chloroplasts of cell s in the apical meristem occurred at 6 t o 24 hours after inoculation of the cotyledon 1eaf. 10. Abscisic acid applied to the cotyledon in a single 20 µ1 dose, elicited a response that closely paralleled the events that took place when Chinese cabbage seedlings were inoculated with turnip yellow mosaic virus. A decrease in the rate of leaf initiation began 1 t o 2 days after application and the inhibition of leaf initiation was preceded by a disturbance in the mitotic index in the apical meristem. 11. Gibberellic acid applied with the eluate from virus-inoculated leaves, was able to overcome the inhibition of leaf initiation. 12. The leaf inhibition assay in Chinese cabbage seedlings is a sensitive bioassay for abscisic acid. The minimum detectable concentration of 3 x M is comparable to those reported for the Commelina stomata1 closure bioassay which could detect 10 -10 abscisic acid (Ogunkanmi et a1 . 1973). / Note: Whole document restricted due to copyright restrictions but available by request use the feedback form to request access

A detailed study of the leaf blotch disease, caused by Mycosphaerella nubilosa (Cke.) Hansf. on Eucalyptus regnans F. Muell (Mountain Ash) and E. delegatensis R.T. Baker (Syn. E. gigantea Hook. F., Alpine Ash) was made. The taxonomy of the fungus was studied(illustrations provided) and a previously unidentified imperfect state (Colletogloeum nubilosum) was described along with the spermagonial state (Asteromella). The histology and mode of infection process by ascospores and conidia was examined using Scanning Electron Microscopy, and wax and epoxy resin sections. The development of the fungus within the host was studied up to the stage of mature pseudothecia. The fungus was isolated from diseased tissue and Koch's postulates were carried out to prove pathogenicity. Detailed development of the leaf blotch and twig canker symptoms was followed in the field and in glasshouse conditions. The effect of substrate and environmental conditions on growth and sporulation of the fungus was studied. Controlled environment chambers and glasshouses were used. Several day/night temperature combinations (12/6, 18/12, 24/18 and 30/24C) as well as several light levels (50, 75, 150, 200, 300 and 650 μE) were investigated. The optimum temperature for infection was 24/18C and least infection occurred at 12/6C. The optimum light level for infection was 300 to 650 μE while the slowest infection was obtained at 50 and 75 μE. Under constant temperature conditions infection was most severe at 18C for both E. delegatensis and E. regnans. Wetness periods under sprinklers of one to seven days were compared with similar incubation periods in polythene bags. One-day incubation produced very little infection compared with 2, 4 or 7 days. Incubation in polythene bags gave better results than sprinklers. Experiments showed that leaf and stem infection was not affected by nutrient levels. Leaf expansion rates for both the eucalypt species were followed for twelve weeks to determine the optimum leaf size for infection. Fungicide trials in the field and in-vitro suggested that several chemotherapeutants may be effective in controlling the disease. Diseased leaves from field trials were assessed for ascospore discharge to determine fungal viability. From this study it was shown that diseased (inoculated)"plants lost significantly more leaves than controls. If conditions were optimum for infection, tips of inoculated plants were killed, leading to stem distortion, branching and reduced growth. In general, the fungus causes severe damage on susceptible Eucalyptus spp. in sapling stages.

Botrytis cinerea and Sclerotinia sclerotiorum are the two most serious pathogens on kiwifruit in New Zealand. Because of the pesticide regulations in some of the countries to which New Zealand exports fruit, total protection from Botrytis stem end rot with current dicarboximide fungicides is not possible. The aim of this thesis was to investigate biological control measures for Botrytis stem end rot and Sclerotinia diseases of kiwifruit. More than 1000 microorganisms, isolated from the leaves and flowers of kiwifruit during spring and autumn, and selected from BCAs reported to be effective against B. cinerea and./or S. sclerotiorum, were tested in vitro for their antagonistic ability against B. cinerea and S. sclerotiorum. Successful antagonists were those that, in dual culture on agar plates, produced a zone of inhibition, an area of browning of the pathogens, or grew rapidly over the pathogens and inhibited their growth. The fifty most promising isolates from the initial screen were tested on fruit for their ability to reduce Botrytis and Sclerotinia fruit rots. Mature kiwifruit were artificallv wounded and dual inoculated with a spore suspension of one of the fifty test organisms and either a conidial suspension of B. cinerea or a mycelial suspension of S. sclerotiorum. Following 8-12 weeks incubation in a cool store, fruit were assessed for Botrytis or Sclerotinia induced rot. Isolates of Bacillus spp., Epicoccum purpurascens, Pseudomonas sp. and Trichoderma. spp. reduced, Botrytis fruit rot from 92% (inoculated control) to 0%.Isolates of Alternaria spp., pestalotia sp. and a non-sporulating isolate also reduced the number of fruit rotting to some extent. Similarly, isolates of Bacillus spp., E purpurascens and Trichoderma spp. reduce d Sclerotinia fruit rot from 100% (inoculated control) to 0%. Isolates of Alternaria spp., Myrothecium verrucaria and Pestalotia sp. were also successful at reducing the level of Sclerotinia fruit rot. It was considered undesirable if potential biological control agents (BCAs) were able to colonize kiwifruit that were to be marketed for human consumption. In order to determine if microorganisms, shown to be effective in preventing Botrytis or Sclerotinia fruit rot, were capable of themselves colonizing fruit, isolations were made from fruit dual inoculated with B. cinerea, S. sclerotiorum and/or one of several BCAs. Strains of the BCAs Bacillus spp., Pseudomonas sp. and E. purpurascens were not found to be saprophytic on fruit. Isolates of Alternaria sp., Bacillus sp., E purpurascens, pestalotia sp., Pseudomonas sp. and T. harzianum significantly inhibited germination and germ tube elongation of B. cinerea conidia in vitro in a nutrient solution, over a 24 h period. For example, the presence of Alternaria alternata A6 spores in a nutrient solution reduced germination of B. cinerea conidia from 100% to 20%. The presence of E purpurascens A77 spores inhibited B. cinerea conidial germ tube elongation from >840 pm (in control conidia) to 27 µm. The presence of any one of the BCAs tested prevented germination of B. cinerea conidia in a non-nutrient water solution, in comparision to germination of up to 86% in controls. A spore or cell suspension of each of the isolates Bacillus sp.M60, E. purpurascens A77 and T. harzianum C65 were spray inoculated onto kiwifruit blossoms produced in vivo in the glasshouse, immediately prior to inoculation of the blossoms with a condial suspension of B. cinerea. Application of the BCAs were completely effective in preventing colonization of blossoms by B- cinerea conidia. The effectiveness of each of the isolates E. purpurascens A77,T. harzianum C65 and either Bacillus sp.M60 or M53 to reduce the viability of sclerotia of B. cinerea and S. sclerotiorum was tested in soil punnets. A spore or cell suspension of each respective BCA was applied to the surface of replicated punnets that were seeded with either B. cinerea or S. sclerotiorum. Following 8 weeks incubation, punnets were harvested and viability of sclerotia assessed. T. harzianum C65 and Bacillus sp. M60 significantly reduced the viability of B. cinerea sclerotia from 8 sclerotia/punnet (control) to 4 sclerotia/punnet. T. harzianum C65 and E. purpurascens A77 caused a significant reduction in apothecia production of S. sclerotiorum, from 2.7 apothecia/punnet (control) to 0.7 apothecia/punnet. Bacillus sp.M8 and E purpurascens A77 were tested for their ability to reduce Botrytis stem end rot and Sclerotinia field rot in a kiwifruit orchard. The isolates tested did not successfully reduce either disease. Possible explanations for this are discussed. In order to monitor the survival of particular isolates of BCAs in the field, a technique was developed to distinguish between individual strains of a BCA species. The polymerase chain reaction (PCR) was utilized to identify DNA polymorphisms within the genome of T. harzianum C65, in comparison with other strains of Trichoderma spp.. A sequence of polymorphic DNA was cloned, sequenced and used as a hybridization probe in southern blotting to enable T. harzianum c65 to be distinguished from other strains of Trichoderma spp.. From the results obtained in this study, it was considered that Bacillus M60, E purpurascens 477 and Pseudomonas M30 were the best isolates for the biological control of Botrytis stem end rot on kiwifruit. Further work to enable application of these isolates as postharvest BCAs is discussed. Of the isolates tested in this study, T. harzianum C65 was considered the best isolate for use against Sclerotinia diseases on kiwifruit. Methods of selecting more effective BCAs against S. sclerotiorum are discussed.

1. When purified turnip ye1low mosaic virus was inoculated mechanically on to Chinese cabbage leaves, using known numbers of virus particles in 0.1 to 1.0 µ1 volumes of inoculum, as few as 10 to 30 particles were required to produce a single local lesion.2. Inoculation of a cotyledon leaf of Chinese cabbage seedlings with turnip yellow mosaic virus produced a rapid transient inhibition in the rate of leaf initiation, so that infected plants developed 0.5 to 1.0 leaf less than healthy plants. 3. The factor that initiated the inhibitory response a t the apical. meristem began moving out of the inoculated cotyledon within 1to 6 hours after inoculation, thus preceding the movement out of the inoculated cotyledon of infectious virus or RNA which was not detectable until about day 5. 4. The transient inhibition of leaf initiation occurred following inoculation with any one of three unrelated viruses, or with infectious turnip ye1low mosaic virus RNA. 5. A factor eluted in an active form from the cut petioles of inoculated 1eaves. 6. It is necessary to inoculate with infectious virus or RNA to initiate the production of the inhibitory factor. 7. No differences were seen in the magnitude or timing of the reduced rate of 1eaf initiation, when the concentration of turnip yellow mosaic virus in the inoculum was varied between 1 µ g/ml and 100 µg/ml. 8. Inoculation of the cotyledons of Chinese cabbage seed1ings with turnip ye1low mosaic virus caused a marked disturbance in the mitotic index a t the apical meristem between 6 and 48 hours. 9. A reduction in the accumulation of starch in the chloroplasts of cell s in the apical meristem occurred at 6 t o 24 hours after inoculation of the cotyledon 1eaf. 10. Abscisic acid applied to the cotyledon in a single 20 µ1 dose, elicited a response that closely paralleled the events that took place when Chinese cabbage seedlings were inoculated with turnip yellow mosaic virus. A decrease in the rate of leaf initiation began 1 t o 2 days after application and the inhibition of leaf initiation was preceded by a disturbance in the mitotic index in the apical meristem. 11. Gibberellic acid applied with the eluate from virus-inoculated leaves, was able to overcome the inhibition of leaf initiation. 12. The leaf inhibition assay in Chinese cabbage seedlings is a sensitive bioassay for abscisic acid. The minimum detectable concentration of 3 x M is comparable to those reported for the Commelina stomata1 closure bioassay which could detect 10 -10 abscisic acid (Ogunkanmi et a1 . 1973). / Note: Whole document restricted due to copyright restrictions but available by request use the feedback form to request access

A detailed study of the leaf blotch disease, caused by Mycosphaerella nubilosa (Cke.) Hansf. on Eucalyptus regnans F. Muell (Mountain Ash) and E. delegatensis R.T. Baker (Syn. E. gigantea Hook. F., Alpine Ash) was made. The taxonomy of the fungus was studied(illustrations provided) and a previously unidentified imperfect state (Colletogloeum nubilosum) was described along with the spermagonial state (Asteromella). The histology and mode of infection process by ascospores and conidia was examined using Scanning Electron Microscopy, and wax and epoxy resin sections. The development of the fungus within the host was studied up to the stage of mature pseudothecia. The fungus was isolated from diseased tissue and Koch's postulates were carried out to prove pathogenicity. Detailed development of the leaf blotch and twig canker symptoms was followed in the field and in glasshouse conditions. The effect of substrate and environmental conditions on growth and sporulation of the fungus was studied. Controlled environment chambers and glasshouses were used. Several day/night temperature combinations (12/6, 18/12, 24/18 and 30/24C) as well as several light levels (50, 75, 150, 200, 300 and 650 μE) were investigated. The optimum temperature for infection was 24/18C and least infection occurred at 12/6C. The optimum light level for infection was 300 to 650 μE while the slowest infection was obtained at 50 and 75 μE. Under constant temperature conditions infection was most severe at 18C for both E. delegatensis and E. regnans. Wetness periods under sprinklers of one to seven days were compared with similar incubation periods in polythene bags. One-day incubation produced very little infection compared with 2, 4 or 7 days. Incubation in polythene bags gave better results than sprinklers. Experiments showed that leaf and stem infection was not affected by nutrient levels. Leaf expansion rates for both the eucalypt species were followed for twelve weeks to determine the optimum leaf size for infection. Fungicide trials in the field and in-vitro suggested that several chemotherapeutants may be effective in controlling the disease. Diseased leaves from field trials were assessed for ascospore discharge to determine fungal viability. From this study it was shown that diseased (inoculated)"plants lost significantly more leaves than controls. If conditions were optimum for infection, tips of inoculated plants were killed, leading to stem distortion, branching and reduced growth. In general, the fungus causes severe damage on susceptible Eucalyptus spp. in sapling stages.

Botrytis cinerea and Sclerotinia sclerotiorum are the two most serious pathogens on kiwifruit in New Zealand. Because of the pesticide regulations in some of the countries to which New Zealand exports fruit, total protection from Botrytis stem end rot with current dicarboximide fungicides is not possible. The aim of this thesis was to investigate biological control measures for Botrytis stem end rot and Sclerotinia diseases of kiwifruit. More than 1000 microorganisms, isolated from the leaves and flowers of kiwifruit during spring and autumn, and selected from BCAs reported to be effective against B. cinerea and./or S. sclerotiorum, were tested in vitro for their antagonistic ability against B. cinerea and S. sclerotiorum. Successful antagonists were those that, in dual culture on agar plates, produced a zone of inhibition, an area of browning of the pathogens, or grew rapidly over the pathogens and inhibited their growth. The fifty most promising isolates from the initial screen were tested on fruit for their ability to reduce Botrytis and Sclerotinia fruit rots. Mature kiwifruit were artificallv wounded and dual inoculated with a spore suspension of one of the fifty test organisms and either a conidial suspension of B. cinerea or a mycelial suspension of S. sclerotiorum. Following 8-12 weeks incubation in a cool store, fruit were assessed for Botrytis or Sclerotinia induced rot. Isolates of Bacillus spp., Epicoccum purpurascens, Pseudomonas sp. and Trichoderma. spp. reduced, Botrytis fruit rot from 92% (inoculated control) to 0%.Isolates of Alternaria spp., pestalotia sp. and a non-sporulating isolate also reduced the number of fruit rotting to some extent. Similarly, isolates of Bacillus spp., E purpurascens and Trichoderma spp. reduce d Sclerotinia fruit rot from 100% (inoculated control) to 0%. Isolates of Alternaria spp., Myrothecium verrucaria and Pestalotia sp. were also successful at reducing the level of Sclerotinia fruit rot. It was considered undesirable if potential biological control agents (BCAs) were able to colonize kiwifruit that were to be marketed for human consumption. In order to determine if microorganisms, shown to be effective in preventing Botrytis or Sclerotinia fruit rot, were capable of themselves colonizing fruit, isolations were made from fruit dual inoculated with B. cinerea, S. sclerotiorum and/or one of several BCAs. Strains of the BCAs Bacillus spp., Pseudomonas sp. and E. purpurascens were not found to be saprophytic on fruit. Isolates of Alternaria sp., Bacillus sp., E purpurascens, pestalotia sp., Pseudomonas sp. and T. harzianum significantly inhibited germination and germ tube elongation of B. cinerea conidia in vitro in a nutrient solution, over a 24 h period. For example, the presence of Alternaria alternata A6 spores in a nutrient solution reduced germination of B. cinerea conidia from 100% to 20%. The presence of E purpurascens A77 spores inhibited B. cinerea conidial germ tube elongation from >840 pm (in control conidia) to 27 µm. The presence of any one of the BCAs tested prevented germination of B. cinerea conidia in a non-nutrient water solution, in comparision to germination of up to 86% in controls. A spore or cell suspension of each of the isolates Bacillus sp.M60, E. purpurascens A77 and T. harzianum C65 were spray inoculated onto kiwifruit blossoms produced in vivo in the glasshouse, immediately prior to inoculation of the blossoms with a condial suspension of B. cinerea. Application of the BCAs were completely effective in preventing colonization of blossoms by B- cinerea conidia. The effectiveness of each of the isolates E. purpurascens A77,T. harzianum C65 and either Bacillus sp.M60 or M53 to reduce the viability of sclerotia of B. cinerea and S. sclerotiorum was tested in soil punnets. A spore or cell suspension of each respective BCA was applied to the surface of replicated punnets that were seeded with either B. cinerea or S. sclerotiorum. Following 8 weeks incubation, punnets were harvested and viability of sclerotia assessed. T. harzianum C65 and Bacillus sp. M60 significantly reduced the viability of B. cinerea sclerotia from 8 sclerotia/punnet (control) to 4 sclerotia/punnet. T. harzianum C65 and E. purpurascens A77 caused a significant reduction in apothecia production of S. sclerotiorum, from 2.7 apothecia/punnet (control) to 0.7 apothecia/punnet. Bacillus sp.M8 and E purpurascens A77 were tested for their ability to reduce Botrytis stem end rot and Sclerotinia field rot in a kiwifruit orchard. The isolates tested did not successfully reduce either disease. Possible explanations for this are discussed. In order to monitor the survival of particular isolates of BCAs in the field, a technique was developed to distinguish between individual strains of a BCA species. The polymerase chain reaction (PCR) was utilized to identify DNA polymorphisms within the genome of T. harzianum C65, in comparison with other strains of Trichoderma spp.. A sequence of polymorphic DNA was cloned, sequenced and used as a hybridization probe in southern blotting to enable T. harzianum c65 to be distinguished from other strains of Trichoderma spp.. From the results obtained in this study, it was considered that Bacillus M60, E purpurascens 477 and Pseudomonas M30 were the best isolates for the biological control of Botrytis stem end rot on kiwifruit. Further work to enable application of these isolates as postharvest BCAs is discussed. Of the isolates tested in this study, T. harzianum C65 was considered the best isolate for use against Sclerotinia diseases on kiwifruit. Methods of selecting more effective BCAs against S. sclerotiorum are discussed.

1. When purified turnip ye1low mosaic virus was inoculated mechanically on to Chinese cabbage leaves, using known numbers of virus particles in 0.1 to 1.0 µ1 volumes of inoculum, as few as 10 to 30 particles were required to produce a single local lesion.2. Inoculation of a cotyledon leaf of Chinese cabbage seedlings with turnip yellow mosaic virus produced a rapid transient inhibition in the rate of leaf initiation, so that infected plants developed 0.5 to 1.0 leaf less than healthy plants. 3. The factor that initiated the inhibitory response a t the apical. meristem began moving out of the inoculated cotyledon within 1to 6 hours after inoculation, thus preceding the movement out of the inoculated cotyledon of infectious virus or RNA which was not detectable until about day 5. 4. The transient inhibition of leaf initiation occurred following inoculation with any one of three unrelated viruses, or with infectious turnip ye1low mosaic virus RNA. 5. A factor eluted in an active form from the cut petioles of inoculated 1eaves. 6. It is necessary to inoculate with infectious virus or RNA to initiate the production of the inhibitory factor. 7. No differences were seen in the magnitude or timing of the reduced rate of 1eaf initiation, when the concentration of turnip yellow mosaic virus in the inoculum was varied between 1 µ g/ml and 100 µg/ml. 8. Inoculation of the cotyledons of Chinese cabbage seed1ings with turnip ye1low mosaic virus caused a marked disturbance in the mitotic index a t the apical meristem between 6 and 48 hours. 9. A reduction in the accumulation of starch in the chloroplasts of cell s in the apical meristem occurred at 6 t o 24 hours after inoculation of the cotyledon 1eaf. 10. Abscisic acid applied to the cotyledon in a single 20 µ1 dose, elicited a response that closely paralleled the events that took place when Chinese cabbage seedlings were inoculated with turnip yellow mosaic virus. A decrease in the rate of leaf initiation began 1 t o 2 days after application and the inhibition of leaf initiation was preceded by a disturbance in the mitotic index in the apical meristem. 11. Gibberellic acid applied with the eluate from virus-inoculated leaves, was able to overcome the inhibition of leaf initiation. 12. The leaf inhibition assay in Chinese cabbage seedlings is a sensitive bioassay for abscisic acid. The minimum detectable concentration of 3 x M is comparable to those reported for the Commelina stomata1 closure bioassay which could detect 10 -10 abscisic acid (Ogunkanmi et a1 . 1973). / Note: Whole document restricted due to copyright restrictions but available by request use the feedback form to request access