P. G. Ayres

On the use of fungicides for experimentation in natural vegetation

The «chemical exclusion» of fungi, using fungicides, is probably the only practical method of studying the overall role of these organisms in natural vegetation. We report here the responses of 19 herbaceous wild species to five fungicide treatments

COMBINED EFFECT OF FOLIAR AND MYCORRHIZAL ENDOPHYTES ON AN INSECT HERBIVORE

Many foliar endophytic fungi deter herbivores from feeding on their host plants, but a previous study has suggested that their deterrent effect may be reduced in the presence of arbuscular mycorrhizae. In our study, factorial experiments determined the effects of infection of perennial ryegrass, Lolium perenne, by the foliar endophyte Neotyphodium lolii [present (+E) or absent (−E)] and the mycorrhizal fungus Glomus mosseae [present (+M) or absent (−M)] on larvae of the noctuid moth Phlogophora meticulosa. When host plants received adequate phosphorus (P), mycorrhizae and the foliar endophyte decreased the survivorship of larvae between their second and fifth instar in an additive manner. Sixth-instar larvae feeding on −M−E plants attained greater mass than those feeding on plants with one or both symbionts. There was an M × E interaction with the total amount of grass consumed per larva. The foliar endophyte increased relative consumption rate and reduced efficiency of conversion of ingested food of larv...

Variation in the responses of litter and phylloplane fungi to UV-B radiation (290–315 nm)

The development of 12 litter and seven phylloplane fungal species was examined from spore germination to colony sporulation across a series of environmentally relevant UV-B doses. For the litter fungi all aspects of fungal development and morphology studied were affected. On the basis of the responses of mycelial extension rate and spore germination to increasing UV-B, the 12 litter fungi were divided into two groups. Group A (Aspergillus fumigatus, Penicillium hordei, P. janczewskii, P. spinulosum and P. purpurogenum) were sensitive to UV-B, with the predicted effects of a 15% ozone depletion resulting in 22–44% reductions in spore germination. Mycelial extension rate on the agar surface was similarly affected, with reductions ranging from 15 to 25%. In contrast group B (Mucor hiemalis, Cladosporium cladosporioides, Leptosphaeria coniothyrium, Trichoderma viride, Ulocladium consortiale, the Verticillium state of Nectria inventa and Marasmius androsaceus) were relatively insensitive to UV-B, with significant, but small, reductions in mycelial extension rate (< 5%) and spore germination (0–22%). Spore production in response to UV-B in the litter species was very variable, reductions ranging from 5% to complete inhibition. Only P. hordei showed a significant increase in spore production in response to UV-B dose. In contrast, in all seven phylloplane species, spore germination was unaffected by increasing dose. Mycelial extension rate was slightly (2–12%), but significantly, inhibited by UV-B for the four phylloplane fungi tested. The contrasting responses of phylloplane and litter fungi to UV-B are discussed along with the implications for resource capture by competing fungal species and the possible effects of UV-B on decomposition processes.

The role of interactions between trophic levels in determining the effects of UV-B on terrestrial ecosystems

Understanding the potential impact of ozone depletion on terrestrial ecosystems is constrained by lack of information on the effects of environmentally realistic UV-B doses on terrestrial organisms other than higher plants. Increasing UV-B may alter interactions between plants and consumers through direct effects on consumer organisms (herbivores, phytopathogens, decomposers, etc.). The effects of increasing UV-B on arthropods are not known. Significant UV-B effects on fungi have been reported, and may be either negative (inhibition of spore germination and mycelial growth) or positive (increased growth, induction of reproductive development and sporulation). However, in many cases consumers are unlikely to be directly exposed to UV-B in the field. In addition, UV action spectra for fungi suggest that this major group may be less sensitive to the effects of ozone depletion than higher plants. Host mediated effects of UV-B on consumers may include alterations in plant chemistry. While secondary metabolites such as phenolics may increase under increased UV-B, this is not invariably the case and evidence that such changes have significant effects on consumers is limited. In particular, there is no evidence that increased UV-B increases resistance of higher plants to fungal pathogens. Indeed, increased UV-B prior to inoculation results in no significant effect or increased disease. Such responses may be attributable to UV-B effects on host surface properties or on compounds other than phenolics. However, such changes are poorly known, and their potential effects on phytopathogens, herbivores or decomposers cannot be assessed. Understanding the effects of UV-B on terrestrial ecosystems is further limited since virtually nothing is known of possible impacts on higher trophic levels, i.e. predators, parasites or pathogens.

The effect of a foliar disease (rust) on the development of Gastrophysa viridula (Coleoptera: Chrysomelidae)

Abstract. 1 Gastrophysa viridula Degeer (Coleoptera: Chrysomelidae) and the pathogenic rust fungus Uromyces rumicis (Schum.) Wint. both occur on leaves of Rumex crispus L. and R.obtusifolius L. Individual stages of beetle development, and egg laying, were compared on healthy and infected leaves of each plant species in the laboratory. Oviposition choice was investigated in the field and laboratory. 2 Beetles reared on infected leaves of each species had greater larval mortality and slower development than those reared on healthy leaves. Although larvae feeding on infected leaves consumed up to 2.5 times more dry weight than those reared on healthy leaves, they had a lower relative growth rate and pupated at a lower weight. These changes were consistent with the reduced nutritive quality of rust‐infected Rumex leaves. 3 Fecundity of beetles reared on infected leaves of both species was considerably reduced. Eggs laid by beetles feeding on infected R.crispus leaves also had a reduced viability. 4 The beetle developed consistently poorer on healthy R.crispus than on healthy R.obtusifolius throughout its life‐cycle. Differences in larval performance were greater between host species than between infected and healthy leaves. 5 Oviposition was similar on infected and healthy R.crispus in both the laboratory and field. However, adults consumed less, and laid fewer eggs on infected than on healthy R.obtusifolius. The pattern of egg laying on different aged leaves was affected by rust infection: a greater proportion of eggs was laid on the older, infected leaves, than on the equivalent aged leaves on the healthy plants. Few larvae survived from eggs laid on rusted leaves in the field.

Interactions Between Rumex spp., Herbivores and a Rust Fungus: Gastrophysa viridula Grazing Reduces Subsequent Infection by Uromyces rumicis

The chrysomelid beetle Gastrophysa viridula and the rust fungus Uromyces rumicis both occur on leaves of Rumex crispus or Rumex obtusifolius. We investigated reciprocal interactions between herbivory and infection. In both Rumex species the amount of leaf eaten by Gastrophysa viridula was not affected by prior rust infection of the same leaf, whereas there was a significant reduction in infection when the same leaves had been previously grazed by Gastrophysa viridula. Herbivory induced an 80 % reduction in pustule density within and immediately around the feeding site within 1 day of feeding damage; it also induced resistance throughout the undamaged portion of the leaf, and a less effective, and slower, resistance in undamaged leaves (...)

The impact of a pathogen (Puccinia lagenophorae) on populations of groundsel (Senecio vulgaris) overwintering in the field. I: Mortality, vegetative growth and the development of size hierarchies

(1) Monocultures of groundsel (Senecio vulgaris L.) were sown in large tubs of soil in the autumn and overwintered in the field. The populations in half of the tubs were inoculated in the autumn with the fungus Puccinia lagenophorae, which causes rust disease. (2) The frequency distribution of rust infection (rust pustules cm-2 leaf area) was highly positively skewed. A few, small plants sustained very high infections. They had smaller leaf areas than larger individuals but the same number of pustules per plant. (3) As a result of severe winter weather, all plants lost leaves and decreased in shoot dry weight. Losses were of similar absolute magnitude in all plants but were relatively more damaging to small plants. (4) High death rates among those plants inoculated in the autumn were attributed to infection of the hypocotyl, which almost invariably killed the host within 1-2 weeks. Heavy infection compromised the capacity of plants to tolerate winter stresses. Small, rust-infected plants were severely injured by the conditions early in winter, which did not harm control plants of comparable size. (5) In both control and infected populations, frequency distributions of shoot dry weight and leaf area became significantly positively skewed following winter injury because small plants suffered disproportionately greater damage. Rust accentuated this trend because it resulted in injury earlier in the winter. (6) In control populations, vegetative regrowth in the spring was rapid until reproduction began. Flowering was delayed in inoculated populations and rates of vegetative growth were greatest in the individuals that had suffered least winter injury.

THE IMPACT OF A PATHOGEN (PUCCINIA LAGENOPHORAE) ON POPULATIONS OF GROUNDSEL (SENECIO VULGARIS) OVERWINTERING IN THE FIELD II. REPRODUCTION

SUMMARY (1) Reproduction during the spring was studied in monocultures of groundsel (Senecio vulgaris L.) overwintered in the field. In the autumn, populations had been either inoculated with a fungal pathogen (Puccinia lagenophorae), which caused rust disease, or left as controls. The effects of rust infection were examined in relation to status within a hierarchical population (assessed by sub-classes based on shoot dry weights). (2) In control populations, the first capitulum buds were produced in late February. The onset of flowering became progressively delayed as status within the hierarchy declined. In all sub-classes of the population, rust infection during the previous autumn delayed flowering in the spring by 1-2 weeks. (3) The percentage of plants within a sub-class that were reproductive (bore buds or capitula) in March and April declined as status in the hierarchy decreased. In classes of comparable status, fewer plants flowered in inoculated than in control populations. (4) In April, sub-classes with low mean dry weights in both populations produced fewer buds and mature capitula. The number of florets per capitulum were also decreased in lower status classes. Rust inhibited all measured components of reproduction in all classes. (5) During May, many large control plants senesced and their reproductive output declined. Thus, most sub-classes of control populations produced a similar number of florets. In contrast, vegetative growth in inoculated populations continued and all measures of reproduction were far greater in the class of highest status than in any other. There was a seven-fold increase in the contribution of this class to floret production (70%) relative to its frequency in the original population. In control populations, there was no more than a two-fold difference between floret production and frequency in the parent population. (6) Infection resulted in no significant change in the percentage of florets that developed into seed or in the viability of seed. (7) Over the duration of this experiment, the potential seed production of control populations (calculated from capitulum production, florets per capitulum and percentage seed set) was >460 000 seeds m-2. Populations that had been rust-infected in the autumn potentially produced 183 000 seeds m-2, a reduction of >60% compared

Survival, growth and reproduction of groundsel (Senecio vulgaris) infected by rust (Puccinia lagenophorae) in the field during summer.

(1) Seedlings of groundsel (Senecio vulgaris L.) were planted into tubs of soil sunk to the level of the normal soil surface at a field site in late spring. Plants in half the tubs were inoculated with rust fungus (Puccinia lagenophorae Cooke). (2) Infection inhibited the expansion of leaves and reduced plant dry weight. The pattern of partitioning of dry weight between organs was little changed. Senescence occurred earlier and more rapidly in rusted plants than in controls. (3) Cumulative production of capitula was inhibited by 43% in infected plants; the number of florets per capitulum was also slightly lower in infected plants than in control plants, resulting in a 46% reduction in overall floret production. (4) Little seedling mortality occurred, but mortality of mature, flowering plants began earlier and was more rapid in inoculated populations than in controls. This greater mortality, combined with the reduced growth of infected individuals, resulted in substantially lower vegetative production in rusted populations. (5) Infection reduced the percentage of plants which flowered, and cumulative production of capitula was some 60% lower in infected than in healthy populations. Cumulative floret production was 64 000 m-2 in control populations, but only 25 000 m-2 in infected populations. (6) Responses of the summer population to rust are contrasted with the previously reported responses to rust of an over-wintering population.

Interactions between Rumex spp., herbivores and a rust fungus: the effect of Uromyces rumicis infection on leaf nutritional quality

1. The rust fungus Uromyces rumicis infects leaves of Rumex crispus and R. obtusifolius causing premature senescence and leaf death. We investigated the effect of infection on nutritional quality of leaves with respect to the chrysomelid beetle Gastrophysa viridula and measured larval survival and growth on rusted and unrusted plants. 2. A smaller biomass of G. viridula was produced and a greater area of leaf consumed per unit biomass gained, by egg batches reared on rusted compared to unrusted R. obtusifolius, although no difference was observed between larvae reared on rusted or unrusted R. crispus. 3. Concentrations of total non-structural carbohydrates were not changed in the pustule and non-infected leaves but were increased in the inter-pustule area. Infection changed the partitioning to different carbohydrates, increasing the proportion of starch in R. crispus and decreasing it in R. obtusifolius. 4. The concentration of total nitrogen, both in the pustule and the inter-pustule area, decreased in both species with time after rust infection. 5. In both species, considerably higher oxalate concentrations occurred in the interpustule area and significantly lower concentrations in the uninfected leaves of these plants. Calcium concentrations altered similarly and were correlated with oxalate concentrations. 6. We conclude that rust infection does not improve the nutritional quality of the leaves for G. viridula and is unlikely to do so for other invertebrate herbivores