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Theory predicts that bottom-up ecological forces can affect community dynamics, but whether this extends to the effects of heritable plant variation on tritrophic communities is poorly understood. In a field experiment, I contrasted the effects of plant genotype (28 genotypes; 1064 plants), aphid density, and the presence/absence of mutualistic ants in affecting the per capita population growth of a specialist aphid herbivore, as well as the effects of plant genotype on the third trophic level. Plant genotype strongly affected aphid population growth rate, explaining 29% of the total variation in growth rate, whereas aphid density and ant-aphid interactions explained substantially less variation (

In an experiment that directly manipulated grassland plant species richness and composition, decreased plant species richness ('diversity') increased pathogen load (the percentage of leaf area infected by species-specific foliar pathogens across the plant community) in 1998. Pathogen load was almost three times greater in the average monoculture than in the average plot planted with 24 grassland plant species, an approximately natural diversity. Eleven individual diseases increased in severity (percentage of leaf area infected by a single disease) at lower plant species richness, and severity of only one disease was positively correlated with diversity. For 10 of the 11 diseases whose severity was negatively related to diversity, disease severity was positively correlated with host abundance, and in six of these cases, species diversity had no effect on disease severity after controlling for the effects of host abundance. These results suggest that increased abundances of individual host species at lower species diversity increased disease transmission and severity. In 1996 and 1997, similar results for a smaller number of diseases sampled were found in this experiment and another similar one. Although the effect of diversity on disease was highly significant, considerable variance in pathogen load remained among plots of a given diversity level. Much of this residual variance was explained by community characteristics that were a function of the species composition of the communities (the identity of species present vs. those lost). Specifically, communities that lost less disease-prone species had higher pathogen loads; this effect explained more variance in pathogen load than,did diversity. Also, communities that lost the species dominant at high diversity had higher pathogen loads, presumably because relaxed competition allowed greater increases in host abundances, but this effect was weak. Among plant species, disease proneness appeared to be determined more by regional than local processes, because it was better correlated with frequency of the plant species' populations across the region than with local abundance or frequency across the state. In total, our results support the hypothesis that decreased species diversity will increase foliar pathogen load if this increases host abundance and, therefore, disease transmission. Additionally, changes in community characteristics determined by species composition will strongly influence pathogen load. Key words: biodiversity and ecosystem functioning; functional composition; fungal pathogens; global change; habitat simplification; host density; macroecology; parasites; plant community; plant pathogens; polyculture vs. monoculture; species richness.

The role of methyl salicylate (MeSA) in the defense response of plants is investigated. MeSA is a major volatile compound produced by tobacco plants injected with tobacco mosaic virus. It is synthesized from salicylic acid. It may act as an airborne signal which activates disease resistance and the expression of genes related to defense in nearby plants and in the healthy tissues of the infected plant.

1 Various resistance phenotypes of L. marginale can be identified according to their responses to nine distinct pathotypes of the rust fungus M. lini. The relative frequency of these phenotypes was monitored on an 8-m x 8-m permanent plot established at Kiandra in southern New South Wales, Australia. 2 Over the period 1986-92 there was a marked change in the resistance structure of this population. This change was particularly associated with a major epidemic of rust occurring in the summer of 1989. The dominance of the host population by three resistance phenotypes prior to the epidemic was subsequently lost. 3 A similar change in resistance structure was detected in random samples taken in 1981 and 1991 from a different part of the same population. The changes in resistance in both the random-sample and the permanent plots were of no obvious adaptive value. 4 The distribution of the commonest resistance phenotypes on the permanent plot originally showed distinct patterns of aggregation which disappeared after the 1989 epidemic. 5 It is possible that the changing frequencies of particular resistance phenotypes could be explained by linkage between resistance genes and other traits that are under more intense selection. Alternatively, the changing frequencies could result from the combined effects of the presence of a pathogen pathotype which is, at least at certain locations, virulent on all host phenotypes, recruitment patterns in the host population and low pathogen transmission efficiencies. Keywords: disease resistance, epidemic, host-pathogen interactions, nonadaptive evolution, pathotypes, resistance genes

1 We investigated patterns of pollinator visitation to plants of the white campion, Silene alba, and related these to floral nectar rewards in healthy female and male plants and in plants diseased with the anther smut disease Microbotryum violaceum (= Ustilago violacea). 2 Pollinators preferred plants with large floral displays, and also preferred males to females and healthy to diseased plants. Male plants consistently produced nectar with higher sugar concentration, thereby offering higher quality floral rewards than either females or diseased plants. 3 Variation in nectar production was also found among individual plants and different plant families, suggesting that more attractive plants may be predisposed to infection since pollinating insects also serve as vectors for this fungal disease. 4 Such patterns of pollinator preference could affect the dynamics of disease transmission within populations by influencing the probability that insect visitors make transitions between diseased and healthy plants. This vector transmitted disease may therefore modify the course of selection on floral traits by imparting a cost to pollinator attraction. Keywords: anther smut disease, nectar, pollinator preference, Silene alba

Populations of feral donkeys, Equus asinus, in northern Australia are limited by density-dependent mortality during the first 6 mo of individuals' lives. A female's ability to raise her offspring successfully increases with age and is dependent on her maintaining high levels of stored mineral nutrients (calcium, phosphorus, and sodium). A low proportion of offspring was successfully raised in a population at carrying capacity where females had low levels of stored mineral nutrients. Females in a growing population that had been reduced to 45% of carrying capacity had high levels of stored minerals and successfully raised a higher proportion of offspring. Females in the population at carrying capacity ingested a species-poor diet (predominantly a grass, Sehima nervosa) containing low levels of nitrogen and mineral nutrients, and high levels of crude fibre. Females in the growing population ingested a more species-rich diet with higher levels of nitrogen and minerals, and less crude fibre. Levels of calcium, phosphorus, and potassium in the faeces of females in the population at carrying capacity were higher than those in the food ingested. The reverse was true for females in the growing population. At the time of sampling (early dry season), 17% of females in the population at carrying capacity and 0% of females in the growing population were eating or had recently eaten clay. Increased salivary secretion and gastrointestinal irritation caused by high-fibre diets (even when lubricated by clay) may be responsible for mineral depletion of females at carrying capacity, and hence population limitation through the inability of females to raise offspring.

The escape hypothesis predicts that isolated individuals of a given plant species will escape their specialized herbivores and so survive better than individuals growing in clumps. This phenomenon could promote plant coexistence. I examined assumptions underlying the hypothesis by considering insect herbivores associated with neotropical Passiflora vines. I found that the hypothesis was probably of only limited value because plants were attacked by a range of herbivore species, most of which were not really specialized. Furthermore, some oligophagous herbivore species colonized isolated P. pittieri more readily than they colonized plants that were near to other P. pittieri plants: isolated plants did not escape herbivory. Because of the activities of herbivores that feed on > 1 plant species, [greater than or equal to] 2 plant species that share herbivores may not show independent patterns of herbivory or independent local distributions.

Compost and compost teas can activate disease resistance genes and induce systemic resistance to diseases in plants. This had been established by Ohio State University researchers. Findings indicated that when plants were treated with plant teas, root rot protection was induced. When plants were germinated in compost, root rot protection was even greater than when the plants were treated with compost teas alone.

Interactions between plants and soil affect plant-plant interactions and community composition. Festuca arundinacea (tall fescue) is often infected with the mutualistic fungal endophyte Neotyphodium coenophialum. Infected (E+) F. arundinacea has been reported to inhibit soil pathogens, nematodes, and mycorrhizal fungi, and to have greater mineral uptake from the soil relative to uninfected (E-) F. arundinacea. In addition, endophyte presence may indirectly influence the soil by altering the plant-plant interactions and the composition of communities. Recent studies have shown that local plant species diversity was reduced in experimental field plots with E+ F. arundinacea compared to plots with E- plants. Endophyte-induced changes, direct or indirect, in biotic or abiotic soil properties could therefore affect subsequent plant growth. We investigated soil-mediated effects of E+ F. arundinacea on E+ and E- F. arundinacea, Trifolium repens, and Plantago lanceolata by growing each in soil collected from the aforementioned field plots seeded with either E+ or E- F. arundinacea. Infection status of F. arundinacea plots had no direct effect on any measured characteristic of plants grown in soil from those plots. However, responses of experimental plants suggest that, by inducing changes in plant community composition, E+ F. arundinacea may indirectly affect the soil. Most notable was an increase in E+ F. arundinacea total biomass when grown in soil previously dominated by Poa pratensis and other grass species compared to biomass when grown in soil previously dominated by E+ F. arundinacea, indicative of an indirect negative feedback on E+ F. arundinacea growth. In addition, Plantago and Trifolium root biomass was significantly higher when grown in soil that previously supported a high proportion of P. pratensis relative to other plant species. Such plant-soil dynamics may have important community-level implications in areas dominated by F. arundinacea. Key words: community structure; endophyte; Festuca arundinacea; negative feedback; Neotyphodium coenophialum; Plantago lanceolata; plant-soil interaction; Poa pratensis; soil community; Trifolium repens.

1 The effects of fungus infection on a natural population of the pseudoannual plant Trientalis europaea were followed for 4 years. 2 The incidence of the disease was low, showed little temporal variation during the period of study and was not affected by ramet size. Disease reduced flowering, fruiting, stolon length and the number and size of daughter tubers, all of which were positively correlated with ramet size. The year-disease interaction was not significant, except for flowering, suggesting little variation in the aggressiveness of the pathogen. 3 Disease reduced survival of ramets to the end of the growing season, although the effect varied with ramet size, and decreased tuber survival both by reduction of tuber size and by reduction of the overwintering ability of tubers of a given size. 4 For two of the three annual transitions the size of the offspring ramets was affected negatively by infection in the previous year. 5 Disease transmission occurred along the stolons of only 31% of the diseased ramets. The probability of disease being shown in the following year decreased with stolon length. 6 Although disease had a detrimental effect on ramet fitness, the low level of incidence and the stability of the clone dynamics in simulation models suggest only a minor role of the disease in population regulation in this species. Key-words: clonal plants, host--pathogen interaction, population, pseudoannual, temporal patterns Journal of Ecology (1999) 87, 995-1004

The hypothesis that individual plants undergo predictable changes in their resistance to herbivory as they mature was experimentally tested. The performance of two species of insects transferred onto different-aged trees of the same naturally occurring clones of narrowleaf cottonwood, Populus angustifolia, showed opposing and significant changes in host resistance as a function of tree age. The gall-formin aphid Pemphigus betae was 70 times as common on mature trees as on juvenile trees. Transfer experiments demonstrated that this pattern is adaptive; survivorship on mature trees was 50% higher than on juvenile trees. The leaf-feeding battle Chrysomela confluens exhibited opposite distribution on host, with densities 400 times as high on juvenile as on mature trees. This is also adaptive; larvae transferred to mature hosts had 50% lower survival and took 25% longer to reach adulthood than those transferred to juvenile trees. These survivorship and performance data and our inability to support the hypotheses of other mechanisms that would produce the observed distributions suggest that the distributions result from the insects' preferences for different-aged hosts. Several basic implications emerge. (1) Development changes in resistance and susceptibility of hosts are important components in determining the distribution of herbivores. (2) A single plant can change rapidly in its resistance traits with age such that a 10-fold change in resistance can occur over a 2-yr period. (3) As trees mature, increased and decreased resistance to different herbivores can produce nonoverlapping herbivore distributions that could be misinterpreted as being the result of competition (e.g., in this study, beetle abundance was strongly, negatively correlated with aphid abundance). (4) While host maturation processes have large effects on within- and between-tree variation in resistance to herbivores, the implications for herbivore population dynamics and community structuring have not been generally appreciated.

The spread of plant viruses at both cellular and whole-plant level can be studied using low-magnification lenses combined with confocal laser scanning microscopy (CLSM). The use of beta-glucuronidase (GUS), as a marker protein, enables the virus to be localized in specific tissues and cells, but this is a destructive process. The green fluorescent protein (GFP) from Aequorea victoria can be used to investigate the viral infection process, and the use of a potato virus X-based vector to express GFP is reported.

Plants with resistance evoke an incompatible interaction in the infected area to fight off viruses. Plants with transgenes with the same homology as the infecting virus can also resist infection by silencing the gene expression. A natural resistance of Kohlrabi (Brassica oleracea gongylodes) to the DNA pararetrovirus cauliflower mosaic virus, involving post-transcriptional gene silencing, is described.

The cloning of maize genes reveals the functioning of disease resistance genes of plants at molecular levels. The maize plant resists the fungal pathogen which infects the plant by degrading the fungal toxin. The plant resistance gene in the tomato is also cloned, which indicates the presence of gene-to-gene resistance. The tomato plant resistance gene identifies the pathogen's avirulence and produces the hypersensitive response. The response triggers the production anti-microbial compounds.

Studies on resistance development in North American arthroped pests prove that resistance does not depend on generation time but may depend on toxin concentration. Earlier analysis of Bacillus thuringiensis suggested that evolution of immunity depended on generation time and not on selection strength and other factors. Generation time affects resistance depending on the association between generation time and selection intensity of the generation.

American farmers are feeling the effects of a consumer movement against genetically modified (GM) foods which has spread across Europe and Japan. The farmers, while appreciating the convenience of the built-in pesticides in GM crops, fear they will not be able to find buyers for their harvests. The American Corn Growers Association has advised its members not to plant GM maize, while the Organic Trade Association is debating how best to protect its members' crops from 'genetic trespass'.

Some observers believe that extensive planting of corn and cotton which has been genetically engineered to produce a toxin made by the soil bacterium Bacillus thuringiensis (Bt) will place great genetic pressure on insects. This will in turn cause resistance to Bt toxin to emerge very quickly. Furthermore, they believe that resistance management plans are insufficient. They claim that if Bt cotton is to continue to be used, it will be necessary to increase refuges in order to ensure that sufficient numbers of non-resistant insects are present.