Simon A. Archer

The spatial distribution of fungi on decomposing alder leaves in a freshwater stream

SummaryTransects were cut from alder leaves incubated in a freshwater stream and plated as quadrats so that fungal isolates could be mapped by reconstruction of each transect. Initially there was fewer than one aquatic hyphomycete colonist per quadrat, but the mode increased to 6–7 then progressively decreased to <1. Numbers of species of aquatic hyphomycetes per quadrat rose and fell similarly with a maximum mode of 3–4, as did species per transect with a maximum of 11 and a diversity of 16, comprising 6 ‘dominant’ species and about 10 ‘occasional’ species. The latter showed no pattern of appearance but the dominant group was established early and persisted in a dynamic equilibrium. Aquatic hyphomycetes were initially randomly distributed but developed progressively into clumped consortia which persisted after peak colonization, declining as leaf degradation became total. Colonies of the most persistent aquatic hyphomycete species were initially discrete,developing into a complex network of overlapping colonies and species, no two of which showed positive association. These complexes broke down to large colonies of a few species and finally to 1–2 small colonies. The pattern of isolates of the 18 genera of other fungi was the reverse of that for aquatic hyphomycetes. Only Cladosporium, Epicoccum and Fusarium were important colonizers. The first two appear to be inhibited by aquatic hyphomycetes, but were found to degrade substrates representative of cell-wall polymers vigorously whereas aquatic hyphomycetes showed varied degradative ability. Leaf transects were examined by S.E.M. and epifluorescent microscopy so that hyphal colonization could be followed at progressive stages of leaf degradation. Bacteria on transects were patchily distributed, the temporal pattern indicating inhibition by aquatic hyphomycetes and colonization of senescent hyphae.

Restriction fragment length polymorphisms of mitochondrial DNA of Phytophthora infestans

Mitochondrial (mt) DNA has been prepared from 24 isolates of Phytophthora infestans from 11 countries and analysed for the occurrence of restriction fragment length polymorphisms using six restriction endonucleases. An insertion of about 2 kilobase pairs has been detected in mtDNA of five of the isolates and the approximate site of insertion located on the restriction map of P. infestans mtDNA. The insertion occurred in four out of eight isolates of A2 mating type, but in only one out of 15 isolates of A1 mating type. The insertion occurred in mtDNA from isolates from Brazil, Egypt, U.K. and U.S.A., suggesting a global distribution. Two further polymorphisms were detected which could have been due to single base changes or very small deletions or insertions. One of these, in mtDNA lacking the insertion, occurred in four isolates which, although obtained from four different countries (Brazil, Ireland, U.K. and U.S.A.), appeared fairly uniform with respect to nuclear markers and may be closely related.

Effect of formulation, application and rain on the persistence of the entomogenous fungus Metarhizium anisopliae on oilseed rape

The effect of simulated rain on the persistence of oil and water formulations of conidia of the entomogenous fungus Metarhizium anisopliae when applied to oilseed rape foliage was investigated, using third instar larvae of the mustard beetle ( Phaedon cochleariae ) as the target host. Rain significantly ( P M. anisopliae but the amount of inoculum removed was influenced by the formulation. Larvae exposed to plants treated with conidia formulated in aqueous Tween, Shellsol T, or sunflower oil/Shellsol T resulted in 55, 82.5 and 72.5%, mortality, respectively. The mortality for these respective formulations was reduced by 42, 57 and 51% if the plants were exposed for 1 h to simulated rain. Laboratory and field studies showed that more inoculum collected beneath plants sprayed with conidia formulated in Shellsol T or aqueous Tween than in the more viscous sunflower/Shellsol T mixture. Mortality studies on leaves taken from field plots suggested that conidia on leaf surfaces could be replenished by repeated application. The number of conidia isolated from field plots was greater where inoculum was applied bi-weekly than once weekly.

The effects of isothiocyanates on the growth of the entomopathogenic fungus Metarhizium anisopliae and its infection of the mustard beetle

Metarhizium anisopliae has potential as a biological control agent. Included among its hosts are certain insect pests of brassica crops. Brassica species produce isothiocyanates, some of which are known to be fungitoxic. In our study, isothiocyanates inhibited both germination and subsequent growth by M. anisopliae in vitro and its ability to infect insects. Conidia were more sensitive than the mycelium to these compounds, the most fungistatic of which were phenylethyl-, 2-chlorophenyl- and allyl-isothiocyanates. Appressorium production in vitro was suppressed by all isothiocyanates except allyl- and propyl-isothiocyanates, which appeared to stimulate appressorium formation. Phenylethyl- and 3-butenyl isothiocyanates, which are present in several of the plant hosts of Phaedon cochleariae, reduced the pathogenicity of M. anisopliae when inoculated insects were exposed to their vapours. These findings have implications for the efficacy of biocontrol of brassica pests by this fungus.

In vitro toxin production by Fusarium solani f. sp. piperis

Fusarium solani f. sp. piperis (teleomorph: Nectria haematococca f. sp. piperis), causal agent of root rot and stem blight on black pepper (Piper nigrum), produces secondary metabolites with toxigenic properties, capable of inducing vein discoloration in detached leaves and wilting in transpiring microcuttings. Production of F. solani f. sp. piperis (Fsp) toxic metabolites reached a peak after 25 days of static incubation on potato sucrose broth at 25 oC under illumination. Changes in the pH of the culture filtrate did not alter the effect of toxic metabolites. However, when the pH was changed before the medium had been autoclaved, a more intense biological response was observed, with an optimum at pH 6.0. Isolates that produced red pigments in liquid cultures were more efficient in producing biologically active culture filtrates than those which produced pink coloured or clear filtrates suggesting that these pigments could be related to toxigenic activity. Detached leaves of seven black pepper cultivars and Piper betle showed symptoms of vein discoloration after immersion in autoclaved and non-autoclaved Fsp culture filtrates indicating the thermostable nature of these toxic metabolites.

The Role of a Bacterial Siderophore and of Iron in the Germination and Appressorium Formation by Conidia of Colletotrichum acutatum

SUMMARY: Pseudomonas sp. isolate UV3, when added to conidial suspensions of Colletotrichum acutatum, stimulated germination and appressorium formation on glass slides and on host surfaces. A siderophore (SA), purified from low-iron culture of the bacteria, stimulated germination and appressorium formation to a significantly greater extent than the bacterial cells alone. The fully chelated siderophore (SAFe) and free ferric iron were inhibitory in vitro to germination and appressorium formation but inhibited only the latter in vivo. The SA is compared with nutrients at similar concentrations, and the role of the phytoplane microflora and associated siderophores is discussed in relation to fungal infection.

RESISTANCE TO DIEBACK DISEASE CAUSED BY FUSARIUM AND LASIODIPLODIA SPECIES IN CACAO (THEOBROMA CACAO L.) GENOTYPES

Fusarium and Lasiodiplodia species invade feeding lesions caused by mirid bugs (Distantiella theobroma [Dist.] and Sahlbergella singularis Hagl.) and inflict serious damage and yield loss to susceptible cacao (Theobroma cacao L.) varieties in West Africa. As it is the fungal invasion rather than the physical feeding injury by mirids that cause dieback and tree death in cacao, a dieback resistance strategy in cacao crop must take into account resistance to these causal agents. Twenty-nine and 15 cacao genotypes were screened in the laboratory and the greenhouse, respectively, for resistance to isolates of Fusarium decemcellulare and Lasiodiplodia theobromae at Imperial College Londons Biological Sciences Campus, UK. Resistance was assessed as the size of necrotic lesions, distance of fungal colonisation in the stem and the proportion of seedlings with dieback symptoms. Genotypic differences were found in both laboratory and greenhouse tests among various cacao genotypes, and the clones showed a wide range of disease reactions from highly resistant to very susceptible. The pathogenicity of F. decemcellulare and L. theobromae were similar in this study, which suggests that a breeding programme for controlling one of the pathogens can have benefit against the other. Direct significant correlations (r = 0.7) were obtained between visual dieback assessment scores and the percentage cross-sectional area of stem necrosis. Moreover, the response of inoculated stem segments corresponded to the reaction of intact plants despite the variation in the used methodology. Three cacao genotypes (CATIE 1000, T85/799 and MXC 67) were resistant or moderately resistant to F. decemcellulare and L. theobromae. These genotypes could be useful sources of resistance to both pathogens and other wilt causing pathogens in cacao.

R.K.S. Wood FRS, 1919–2017

In May of this year (2017) the plant pathology community lost one of its most visible and respected practitioners of the twentieth century. Few plant pathologists, or plant scientists more generally, whose career spanned the middle part of the twentieth century could be unaware of Ronald Wood (sometimes Ron to his closest contempories, but widely known within the plant pathology community simply as RKS) through his prolific publication record, presentations at conferences, books and reviews. Starting in the late 1940s Ronald Wood published extensively in botanical and other journals. He was one of the first to address seriously the question of how plants either resisted or succumbed to microbial infection, and the parallel questions about what qualities enabled certain microorganisms to incite disease in a plant whereas others, closely related, could not. Tied in with this is the question of what controls hostpathogen specificity, especially that seen on an exquisitely fine scale amongst biotrophic plant pathogens. RKS was an early advocate of ‘looking under the bonnet’ to study the physiology and biochemistry of the interaction between plants and their pathogens. The term ‘Physiological Plant Pathology’ was coined to describe this, possibly not by RKS himself but it was indeed the title of his 1967 book, a highly detailed account of the state of the science at that time. The term spawned an eponymous journal a few years later while the subject itself has slowly morphed into molecular plant pathology as the tools of investigation have become more sophisticated. The state of the subject now allows extremely detailed dissection of the molecular signalling between plant and microbial adversaries but the underlying questions being tackled are much the same as those framed by RonaldWood some 50 years ago. Ronald Karslake Starr Wood was born in Ferndale, south Wales in 1919, into a coalmining family. Few who knew him only later in life would be aware of his humble origins or his Welsh accent as a youngster. Ronald evidently excelled at school and was a beneficiary of both the grammar school and the government scholarship systems which allowed poor pupils to attend university. Despite pressure from his school to attend a local university (a curb on ambition sadly still evident in some schools today) RKS opted for London and went up to Imperial College to read Botany in 1937. His period as an * Simon Archer s.archer@imperial.ac.uk

Changes in the Enzyme Activities of Pisum sativum (Garden Pea) when Infected with Pseudomonas syringae pv. pisi

In the host pathogen interaction between Pisum sativum and Pseudomonas syringae pv. pisi the levels of the two enzymes s 1,3 glucanase and chitinase change after infection. In three pea cultivars examined: Fortune PS 890751, Kelvedon Wonder PS 91029 and Early Onward PS 89074, the levels of the two enzymes increased in intercellular fluids (IF) after infection with race 1 (299A) and race 2 (202) of the pathogen when compared with the healthy plant. The s1,3 glucanase activity was found in three isozymes with isoelectric points of 9.0, 7.8 and 4.6 in Fortune (F) and Kelvedon Wonder (K). The differential cultivar Early Onward (E) which shows hypersensitivity when infected with race 2 of the pathogen and susceptibility when infected with race 1 has four s1, 3 giucanases of isoelectric points 9.0, 8.4, 7.8 and 4.6. Western blotting of intercellular fluids and acidic (pH 2.3) and basic (pH 8.3) extracts showed that Fortune produced a 21 kDa protein which was detected by the s1,3 glucanase and the chitinase/lysozyme antisera.This 21 kDa protein was present both in the susceptible reaction (Fortune x R6) and the resistant reaction (Fortune x R1, Fortune x R 2). In the Early Onward resistant reaction (Early Onward x R2) and the susceptible interaction (Early Onward x R1, Early Onward x R6) a 29 kDa protein was detected by the s1,3 glucanase antisera and the chitinase/lysozyme antisera in the intercellular fluids and the acidic and basic extracts. The universally susceptible cultivar Kelvedon Wonder produced a protein of the same molecular weight (29 kDa) as Early Onward along with another protein of 33kDa detected by the chitinase/lysozyme antisera.