Epiphytic life is the main characteristic of the life cycle ofPseudomonas syringae pv.pisi,pea bacterial blight agent

Pseudomonas syringae pv.pisi, pea bacterial blight agent, is seed-transmitted. Some aspects of its life cycle and its biology were investigated. The colonization of pea plants obtained from naturally infected seeds was studied in natural conditions while high populations of bacteria developed on plants showing no symptoms. Two streptomycin-resistant mutants were used to study the epiphytic life of the pathogen. Populations were monitored in different host-parasite compatibilities. When race 2 or race 6 of the pathogen was surface-inoculated on susceptible cultivars, a decrease of population size was observed during the following one to three days but was followed by an increase to levels 1000 times greater than the initial number detected, without symptoms for most of the plants. When race 2 was surface-inoculated on resistant genotypes or race 6 on non-host plants, bacteria did not multiply but population levels slightly decreased.Pseudomonas syringae pv.pisi shows a resident phase and its development is race-specific. Weeds collected in naturally contaminated pea fields, diseased or not, often harboured the pathogen but with levels smaller than those observed on peas. Pea crop debris and volunteers kept high levels of bacteria for at least eight months after the harvest of a diseased crop. As long as two pea crops are not grown one after the other in the same field, it is unlikely that debris and volunteers will act as an important inoculum source. The development of this pathogen during the growing season is considered as an important parameter to take into account for controlling the disease through seed health testing.     

深圳碧海湾高尔夫球场草坪春季杂草及防除策略研究

对深圳碧海湾高尔夫球场草坪进行调查,发现春季草坪杂草有95种,分属26科80属。阔叶杂草有75种,占杂草总数的78.95%;半日子叶杂草20种,占杂草总数的21.05%。对球道和长草区或边坡的杂草发生特点分别进行分析。针对杂草的发生来源和特点,提出了草坪杂草的防除策略。     

云南中亚热带禾本科高产饲草引种试验

在云南中亚热带试验点,对8个禾本科饲草栽培种进行引种栽培试验,结果表明:墨西哥类玉米、8493墨西哥类玉米、大力士甜高粱、先锋高丹草和M-8IE甜高粱5个栽培种干物质产量高,适应性强,适宜在相似气候条件地区推广种植。     

Effects of different cultivated or weed grasses, grown as pure stands or in combination with wheat, on take-all and its suppression in subsequent wheat crops

Grass species were grown in plots, as pure stands or mixed with wheat, after a sequence of wheat crops in which take-all ( Gaeumannomyces graminis var. tritici ) had developed. Annual brome grasses maintained take-all inoculum in the soil as well as wheat (grown as a continuous sequence), and much better than cultivated species with a perennial habit. Take-all developed more in wheat grown after Anisantha sterilis (barren brome) or Bromus secalinus (rye brome), with or without wheat, than in continuous grass-free wheat in the same year, where take-all decline was apparently occurring. It was equally or more severe, however, in wheat grown after Lolium perenne (rye-grass) or Festuca arundinacea (tall fescue), despite these species having left the least inoculum in the soil. It was most severe in plots where these two grasses had been grown as mixtures with wheat. It is postulated that the presence of these grasses inhibited the development of take-all-suppressive microbiota that had developed in the grass-free wheat crops. The effects of the grasses appeared to be temporary, as amounts of take-all in a second subsequent winter wheat test crop were similar after all treatments. These results have important implications for take-all risk in wheat and, perhaps, other cereal crops grown after grass weed-infested cereals or after set-aside or similar 1-year covers containing weeds or sown grasses, especially in combination with cereal volunteers. They also indicate that grasses might be used experimentally in wheat crop sequences for investigating the mechanisms of suppression of, and conduciveness to, take-all.     

Plant Host Range of Verticillium longisporum and Microsclerotia Density in Swedish Soils

Verticillium longisporum is a soil-borne fungal pathogen causing vascular wilt of Brassica crops. This study was conducted to enhance our knowledge on the host range of V. longisporum. Seven crop species (barley, oat, oilseed rape, pea, red clover, sugar beet and wheat) and five weed species (barren brome, black-grass, charlock, cleavers and scentless mayweed) all common in southern Sweden were evaluated for infection by response to V. longisporum. Oat, spring wheat, oilseed rape, scentless mayweed and charlock inoculated with V. longisporum in a greenhouse showed stunting to various degrees close to the fully ripe stage. Based on the extent of microsclerotia formation, explants were separated into four groups: for pea and wheat, <5% of the samples had formed microsclerotia; for scentless mayweed, 5–10%; for oat, 10–20%; and for charlock and oilseed rape >80%. The results suggest that plant species outside the Brassicaceae can act as reservoirs of V. longisporum inoculum. Soil inoculum densities in nine fields were monitored over a period of 12 months, which ranged from 1 to 48 cfu g⁻¹ soil. Density of microsclerotia was lowest just after harvest, reaching its maximum six months later. No significant correlation between inoculum density in soil and disease incidence on oilseed rape plants was found. However, the data suggest that a threshold of 1 cfu g⁻¹ soil is needed to cause disease on oilseed rape. Species identification based on microsclerotia morphology and PCR analysis showed that V. longisporum dominated in soil of seven, and V. dahliae in two of the nine fields studied.     

The Potential Benefits of Weeds with Reference to Small Holder Agriculture in Africa

Weed control is one of the most important crop protection activities undertaken in both intensive and low-input farming systems. However, even under intensive systems, crop protection which is less dependent on pesticides may require that weeds be managed to obtain a balance between crop and non-crop vegetation to encourage an increase in natural enemies of crop pests. In the low-input farming systems which sustain much of the rural population of Africa, weed control is usually done by hand and clean weeding is often beyond the labour resources of the farming family. The vegetational diversity of peasant agriculture in Africa to which weeds make their contribution, helps to decrease the risk of disease and pest epidemics. In addition to the pest control benefits of a diverse agroecosystem, weeds contribute to the resource base of the rural community, providing a source of secondary foods, medicines and insecticides. Weed control within an integrated crop protection system appropriate to the needs of the resource-poor farmer, requires that weeds are managed in such a way that their biodiversity is maintained and the more useful species retained within the field or field margin. Those weeds with high food potential or which have pesticidal or medicinal properties might be deliberately encouraged within the crop or field margins. Certain weed species may harbour important pests or diseases of local crops and therefore should be selectively removed. The paper reviews and discusses the literature on the beneficial and deleterious effects of weeds and argues for a weed management strategy which balances the effects of weed competition on crop production with the ethnobotanical and pest control attributes of individual weed species and weed communities.     

Potential allelopathic influence of certain pasture weeds

Laboratory and growth chamber research was conducted to determine the potential allelopathy of tissue harvested from certain weeds commonly found in pastures. Results of bioassays indicated that the weeds dogfennel and mayweed chamomile are potentially allelopathic to alfalfa and Italian ryegrass seedlings. Leaf-tissue extracts from the two weed species reduced seedling growth more than stem and root extracts. In most experiments alfalfa seedlings were influenced more than Italian ryegrass seedlings and seedling growth was influenced more than seed germination. Leaf tissue from the weed species mixed into potting soil at a concentration as low as 0.25% (w/w) significantly reduced alfalfa and Italian ryegrass plant development and foliage production. Extract and tissue concentration used in this research were estimated to be similar to concentrations expected to occur within the pasture ecosystem and below concentrations that would result in hypertonicity in the bioassays.     

北京地区草坪杂草防治的初步研究

在摸清北京地区不同类型草坪中不同杂草种类的基础上，选择了一定效果的除草剂对这些优势种进行了２个生长季节的防治试验。结果表明，不同除草剂对草坪草敏感程度和防治效果不一样。在使用除草剂防治草坪杂草时一定要谨慎，否则将出现较大药害。     

Direct infection of potato tubers by the root parasite Orobanche aegyptiaca

Parasitic plants of the Orobanchaceae are known as obligate root parasites that develop haustoria that connect to roots of various host plants. This article describes, for the first time, a case where the root parasite successfully connected to potato tubers, i.e. to the swollen portion of an underground stem rather than to a root. The rhizosphere of Orobanche aegyptiaca and of its host Solanum tuberosum (potato) was carefully examined. In anatomical studies, the adventitious roots were directly connected to potato tubers. Numerous secondary haustoria, which developed along the adventitious roots in close vicinity to the potato tuber, penetrated the tuber epidermis and the perimedullary tuber parenchyma and developed xylem strands that are presumably connected to the minor xylem strands within the tuber cortex. These findings indicate that parasites of the Orobanchaceae that normally attack host roots may also parasitise underground stem tubers.