On difficulties with allelopathy methodology

Summary. The main techniques and the common experimental methods used in studying allelopathy are reviewed. Problems associated with both methodology and conclusions drawn based on certain techniques are discussed. The accuracy obtained using these techniques is evaluated and show that some of the techniques could be a misleading tool. Some changes are suggested.     

The significance of VA-Mycorrhiza to future arable farming in the Netherlands

Present conditions of high inputs and narrow rotations render the importance of vesicular-arbuscular mycorrhiza (VAM) in arable farming in the Netherlands rather low. Changes expected in the near future under the influence of economic pressures and environmental problems may change this situation. This is discussed in the light of some such expected changes in Dutch arable farming.     

Designing resistance management programs: How can you choose?

A list of often-discussed resistance management tactics is critically reviewed. Alteration of insecticides across generations, non-persistent formulations, and the use of pesticides conferring only low magnitudes of resistance seem to be among the most promising tactics for general use in resistance management. Justifying the first two choices usually requires little more than data on product persistence in terms of toxic effect on susceptible individuals under field conditions. The last tactic requires additional information on resistance ratios for resistant and susceptible genotypes under field exposure. The efficacy of pesticide mixtures is highly dependent on equal persistence of both compounds used in the mixture. The number of genes involved in resistance and the fitness disadvantages they may confer in the absence of use appear to be of relatively little significance in choosing management tactics.     

The use of entomogenous fungi for pest control and the role of toxins in pathogenesis

Fungi have been suggested as agents for the biological control of insects for over a century, but their use remains extremely limited. This paper examines the biology of entomogenous fungi, highlighting their need for moisture, and reviews studies on the Entomophthoraceae, Beauveria bassiana, Beauveria brongniartii, Metarhizium anisopliae and Verticillium lecanii. It is suggested that current studies should be directed towards the control of pests living in humid micro-climates and that moisture-retaining formulations should be developed to reduce the reliance of fungi on moisture. Many entomogenous fungi produce insecticidal toxins in submerged culture and these compounds are of interest as a source of new toxophores. The most extensively studied toxins are the cyclic depsipeptides from M. anisopliae, termed destruxins, and their possible role in pathogenesis is discussed. Beauveria bassiana also produces the cyclic peptides termed beauvericin, beauverolides, and bassianolide. Toxins produced by V. lecanii, and the Entomophthoraceae are also described.     

Diclofop-methyl tolerance in cultivated oats (Avena sativa L.)

The feasibility of achieving selective wild oat (Avena fatua L.) control in a stand of the Australian oat (Avena sativa L.) cv. Savena I was examined. Diclofop-methyl was applied at rates of 0·4–0·7 kg a.i. ha^?1 at the three- and five-leaf stages of A. fatua (84–132 culms m²). Although all rates of diclofop-methyl caused initial chlorosis and necrosis to the crop, the subsequent control of A. fatua permitted increased crop tillering. Avena fatua control and crop yield response were maximized when diclofop-methyl was applied at the three-leaf stage. Crop grain yield was increased by as much as 32% and 22% during 1984 and 1985, respectively. In the absence of A. fatua competition, crop shoot dry weight at ear emergence was reduced only where diclofop-methyl was applied at 0·6 or 0·7 kg ha^?1. However, final grain yield was not reduced by any treatment. Only 11 of 240 cultivars or lines tolerated diclofop-methyl, and none reached the level attained by Savena I and a closely related Australian line [(Irwin × (West × New Zealand Cape/42)) × West]/24. The nature of the inheritance of diclofop-methyl tolerance was examined (1983–1985) by crossing and backcrossing (BC) Savena I with four diclofop-methyl susceptible, but agronomically superior, lines. Inheritance of diclofop-methyl tolerance appeared to be controlled by two genes, with susceptibility being dominant to tolerance in F₃, BCIF₂ and BCIF₃ lines.     

Pythium oligandmm associated with a cropping disorder of Agaricus bisporus

Pythium oligandrum was consistently isolated from black patches of mushroom compost from two farms. When used in experiments it inhibited the growth of mycelium of Agaricus bisporus and this effect was enhanced in the presence of a compost supplement which contained high levels of nitrogen. The common name pythium-black compost is proposed for the problem.     

A serious disease of seedling rambutan caused by Pseudocercospora nephelii sp. nov.

Pseudocercospora nephelii is described from necrotic lesions on leaves of Nephelium lappaceum seedlings in Brunei, sometimes causing complete defoliation and death. The pathogen also occurs in Sabah and Selangor in Malaysia. Recommendations for control include treatment with mancozeb or benomyl.     

Review—mechanism of action of herbicidal and fungicidal compounds on cell membranes

The eukaryotic cell membrane is thought to consist of a mobile bilayer of phospholipid, sometimes intercalated with sterols, in which peripheral and transmembrane proteins are embedded. This provides a model whereby the mode of action of many fungicidal and herbicidal compounds can be rationalised and understood. Some compounds, such as the polyene antibiotics, steroidal saponins and certain phytoalexins, induce membrane malfunction by direct insertion, often complexing with vital components. Others, such as paraquat and the nitrodiphenyl ether herbicides, cause membrane damage by inducing lipid peroxidations. Furthermore, there are many compounds, including the sterol-biosynthesis-inhibiting fungicides and the aryloxyphenoxypropionate and cyclohexanedione herbicides, which interfere with the biosynthesis of membrane components. Because membrane structure is fundamentally similar in eukaryotes, it is important to discover the reasons for any differential toxicity displayed by these compounds and, if necessary, to find ways of optimising desirable patterns of selectivity.     

水稻品种苗期阶段对白叶枯病的抗性研究

 根据水稻品种各叶期对白叶枯病的抗性表现和强弱程度可分为四类:(1)全期抗病型;(2)全期中抗型;(3)苗感成株抗病型;和(4)全期感病型。可以看出,水稻品种除少数苗感成抗型外,其余三种抗性类型苗期与成株期的抗性反应是一致的。由于中抗、成抗和感病品种各叶期的抗性有随着叶位的升高而抗性增强的趋势,不同抗性品种在苗期阶段抗感对比均较成株期明显。因此,水稻品种对白叶枯病的抗性鉴定采用苗期鉴定方法是可行的。影响水稻品种苗期抗性的因素是:苗龄(叶期)、观察时期、品种与菌株的组合等。     

Sources, survival and transmission of Cladosporium allii and C. allii-cepae, leaf blotch pathogens of leek and onion

Neither Cladosporium allii nor C . allii-cepae was detected in seed samples of 46 commercial cultivars of leek and nine of onion.
In glasshouse tests where flower inflorescences of onions and leeks were dusted with conidia of C. alliicepae and C. allii , respectively, few onion seeds and no leek seeds became contaminated. In field crops of leeks some spathes and inflorescences became diseased and gave 0 002% C. allii contaminated seeds. No infected onion seeds were obtained from naturally-infected plants in the field.
Conidia of both fungi survived for less than 6 weeks in sterile and non-sterile soil (during which time the temperature varied from -8 to +11 C); pseudothecial bodies survived longer.
Both pathogens survived for 3 months in debris of onion or leek on the surface of soil in pots under similar conditions. In debris buried at a depth of 7 cm in sterile or unsterile soil they survived for 2 months. Although C. allii survived only 1 month in debris on the soil surfaces following a leek crop, conidia of the fungus were trapped from the air above the area for at least 6 months and healthy leeks transplanted into this area developed the disease. No leaf blotch developed in leeks grown on land which had not previously borne that crop.