Pseudomonas andropogonis was recorded for the first time as the cause of lesions on blueberry (Vaccinium sp.). carob (Ceratonia siliqua L.), Gypsophila paniculata L. G. elegans Bieb. and statice (Limonium sinuatum (L.) Mill). Cross‐inoculation studies with 13 isolates from 10 host plants showed that sorghum, sweet corn, vetch, carnation and Gypsophila elegans were hosts in common, whereas clovers (Trifolium repens and T. pratense) were infected only by isolates from clover, Gypsophila paniculata and vetch. Seventeen isolates subjected to bacteriological characterization tests formed a uniform phenotype. A single isolate from sorghum produced an antimetabolite inhibitor of Escherichia coli B which was reversed by L‐glutamine. but not by other amino acids.

Xanthomonas campestris pv. malvacearum was transmitted from infested seed to the cotyledons of cotton cv. Deltapine 61 seedlings at 28°C and relative humidities (RH) of 90% or 73%. A resident population was present on the first and second true leaves but not on the third true leaf of plants at either RH. There were smaller numbers of resident bacteria on fewer leaves of plants at the lower RH than on plants at the higher RH. Cotton plants grown from infested seed at 25°C and 30°C and incubated at 100% RH at different stages of growth developed bacterial blight on leaves that were in bud or partly expanded when incubated. Resident cells of this pathogen can thus invade susceptible leaves when conditions are favourable for infection. Bacterial blight developed on more plants at 30°C than at 25°C. In a field trial, X. campestris pv. malvacearum transmitted from seed was present as resident bacteria on the third leaf from the growing point during the vegetative development of the plant. Resident bacteria, which infected young leaves during rainy periods, were isolated from the bacterial blight lesions which subsequently developed.

Populations of Pseudomonas solanacearum biovar 3 were monitored in a clay loam soil sampled from the root zone of infected tomato plants during 1978, 1979 and 1980. Soil numbers increased during symptom development and declined with the death of infected plants. The decline in population size in the soil was continuous where no cover crop was planted between the autumn and spring crops. This decline in population size was interrupted, however, following the planting of an oats cover crop numbers decreased with the ploughing under of the oats. Rainfall was associated with high soil numbers but soil temperature did not appear to directly affect population size. Soil populations in the root zone of susceptible tomato plants cultivar Floradel reached a maximum 1000-fold greater than in soil from the root zone of a resistant line. P. solanacearum survived in bare fallow soil for 21 months. Tomatoes planted 2 months later wilted rapidly.

Bacterial can ker caused by Corynebacterium michiganense subsp. michiganense (Smith 1910) Jensen 1934 (3) is an important disease in both staked and ground tomato (Lycopersicon esculentum Miller) crops in Queensland. The tomato cultivars grown commercially have no resistance to this bacterium and control has been based on the planting of pathogenfree seed. Although the use of such seed has reduced disease Incidence, outbreaks of bacterial canker occur. Growers are concerned that the pathogen survives in the soil between consecutive crops thus negating the benefit of planting pathogen-free seed. In the United States. Echandi (5) reported that C. michiganense subsp. michiganense survived for only two weeks when the organism was introduced into the soil as free cells. He also found that it was able to overwinter In decomposing plant material. Survival in host tissue in soil exposed to winter freezing was reported aiso by other workers (6, 7, 9). Winter freezing comparable to that in the United States does not occur in Queensland so the pathogen buried in the soil in plant debris from the previous crop is affected by different environmental factors. The work reported here investigated the survival of C. michiganense subsp. michiganense in decomposing tomato tissue buried in the field and in soil held at two constant moisture regimes. Soil types used were representative of two important tomato growing districts in Queensland. They were a ciay ioam from the Redland Bay district in south-east Queensland and a granitic sandy loam from Applethorpe on the Granite Belt of Southern Queensland. Artificially inoculated tomato plants provided the infected plant material for this experiment. A pathogenic mutant (1079SR) of C. michiganense subsp. michiganense resistant to 1,000 Jl g mlstreptomycin sulphate was used to inoculate the tomato plants. Growth from a 48 h culture on glucose yeast extract calcium carbonate agar medium (4) containing 500 Jl g ml1 streptomycin sulphate (GYCASS) was suspended in sterile distilled water and the concentration adjusted to 10' viable cells ml• Plants of the cultivar Floradade raised In the glasshouse were inoculated at the fourth leaf stage by excising the cotyledon at the node with a scalpel dipped in inoculum. Inoculated plants were placed on the glasshouse bench and symptoms developed two weeks later. Plants were harvested at the 2nd inflorescence stage. The roots, leaves and apical 4 em of the stems of the diseased tomato plants were discarded and the remaining stem cut into 4 em lengths. Four stem pieces were placed in 6 x 6 em square nylon mesh bags which

The survival of Pseudomonas solanacearum biovars 2 and 3 in three soils, a Nambour clay loam, a Beerwah sandy loam and a Redland Bay clay, was compared at pressure potentials of −0.003, −0.05 and −0.15 kPa. The soils were inoculated with mutants of P. solanacearum biovars 2 and 3, resistant to 2000 μg streptomycin sulphate ml−1 and their survival measured every 6 weeks for 86 weeks in the clay loam and clay and for 52 weeks in the sandy loam. Soil populations declined with the initial drying necessary to bring the soil moisture to the specific pressure potentials; the initial counts for biovar 2 varied between 0.20 and 2.00 × 109 cfu g−1 soil and for biovar 3 between 0.17 and 1.29 × 109 cfu g−1 soil.

SUMMARY Pseudomonas solanacearum was transmitted from contaminated seed to the cotyledons of capsicum (Capsicum frutescens) at 92, 73 and 60% r. h. and to the cotyledons of tomato (Lycopersicon esculentum) seedlings at 92% r. h. Subsequent epiphytic colonisation of the true leaves of capsicum occurred at 92 and 73% r. h. An increase in the population on capsicum cotyledons was detected at 92 and 73% r. h. but only at the higher r. h. was an increased population of P. solanacearum associated with the colonisation of the true leaves. Lesions developed on the true leaves of capsicum at 92% r. h. Transmission of P. solanacearum from capsicum seeds was affected by inoculum concentration, occurring at an infestation level of ∽ 103 propagules seed‐1 but not at 50 propagules seed‐1. Pseudomonas solanacearum was detected on the cotyledons of capsicum held at 98% r. h. after germination of seed in soil infested with 2 × 108 propagules g‐1 soil; lesions were detected on cotyledons 11 days after planting and invasion occurred in 10 stems and one root of the 20 seedlings sampled. The movement of the pathogen from the soil to the seedlings was affected by the level of soil infestation. Pseudomonas solanacearum was detected in four pairs of cotyledons, two stems and one root of the 36 seedlings sampled from soil infested with 107 propagules g‐1 soil but it was not isolated from seedlings sampled from soil infested with 105 or 4 × 103 propagules g‐1 soil. Leaf and stem prints demonstrated the epiphytic nature of this organism on the cotyledons and stems of seedlings.

Effect of Soil Moisture on Decline of Pinus radiata

The major proportion of Australia's economically important bacterial plant pathogens are seed-borne. Seed transmission effectively introduces the pathogen into new areas, enables them to survive in the absence of the host, selects and disseminates host-specific strains and randomly distributes the organism through plant po~lations as foci of infection (Baker, 1972). Seed transport does not ensure the transmission of the organism. Without the occurrence of su itable host-environmental conditions, the actual transmission of the pathogen from the seed to the seedling may not take place. Bacterial pathogens are transmitted by the seed in several ways. They may accompany the seed in trash or plant debris. Leaf pathogens such as Pseudomonas phaseo/ico/a are commonly transmitted in this way. Pathogens contaminate the seed coat and become lodged in cracks in the seed coat. Contamination frequently occurs where seed lies beneath a lesion on a pod, or during extraction of seed from fruit where the pulp is infected by the pathogen. The bean pathogens Xanthomonas phaseo/i, P. phaseo/ico/a and P. syringae contaminate bean seed following pod infection. Pathogens are carried internally in the seed as a result of systemic infection of the plant by the pathogen with the invasion of the developing seed by way of the vascular elements. Xanthomonas campestris can be internally borne in cabbage seed. Transmission of the bacteria from infested seed takes place during contact between the emerging cotyledon and the contaminated seed coat. The organism enters the seedling through the stomata, hydathodes, or injured c~lls. Where the pathogen is borne internally, the organism is carried with the emerging cotyledons, and although the young seedling becomes systemically infected, with little chance of survival to maturity, it acts as a focus of inoculum for dissemination to neighbouring plants. The three bacterial pathogens of beans, P. phaseo/ico/a (halo blight), P. syringae (bacterial brown spot) and X. phaseo/i (common blight) are all seed-borne. The bean seed certification scheme operating in Queensland ensures the production of seed free of P. phaseo/ico/a and X. phaseoh This scheme has been successful because all the certified seed is produced in the dry tropics under furrow irrigation where the average rainfall is approximately 4 inches during the 4 month growing period. The conditions during the whole of this period are unsuitable for bacterial transmission and if a primary infection site occurs it is most unlikely that the organism will be disseminated within that plant or to other plants and the seed produced from the crop should escape contamination by the bacterium. Two reports from overseas workers stated tha~ a very low lev~1 of seed infection, e.g., 0.02% and 0.006Yo respectively, IS sufficient to initiate an epidemic of halo blight. Three bacterial pathogens occur on soybeans in Queensland. These are X. phaseo/i var. sojense (bacterial pustule), P. g/ycinea (bacterial blight) and P. tabaci (wildfire). All three organisms are seed-borne. They are transmitted from one season to the next in a low percentage of the seed and the foci of primary infection and the subsequent dissemination have been observed each year in varietal trials.

Bacterial spot, Xanthomonas pruni, is a serious disease of stone fruit in the Granite Belt district of south-eastern Queensland. The Queensland isolates of X. pruni correspond to those described from overseas.