Reservoir Of Inoculum Research Articles

Natural Occurrence of Phytophthora infestans on Black Nightshade (Solanum nigrum) in Wales.

There is only one published record of natural infection of black nightshade (Solanum nigrum L.) by Phytophthora infestans (Mont.) de Bary in England (3) and none from Wales. In August 2001, brown, necrotic leaf lesions with pale green margins were found on black nightshade weeds in a potato trial naturally infected with P. infestans at Henfaes Research Centre, University of Wales, Bangor. Although the plants were low growing with large, succulent leaves 4 to 5 cm long instead of having a more erect habit and smaller leaves, their identity was confirmed as S. nigrum; their atypical appearance may relate to the known phenotypic plasticity of this species (4). Infected leaflets incubated in moist chambers produced sporangia typical of P. infestans, and zoospores were released after chilling in water. Five isolates obtained from leaf fragments had growth on rye agar that was indistinguishable from that of P. infestans from potato. Detached leaflets of S. nigrum and S. tuberosum cv. Green Mountain inoculated with the S. nigrum isolates developed sporulating lesions under high humidity in 7 to 10 days; uninoculated controls remained symptomless. Inoculation of attached leaves of 10 potted S. nigrum plants resulted in seven plants developing necrotic lesions with a few sporangia 10 to 14 days later; sporulation developed mainly on lower leaves of plants that were older or had senesced. The remaining plants developed necrotic lesions with no sporulation, and P. infestans was reisolated from sporulating and nonsporulating lesions. All isolates were A1 mating type, metalaxyl-sensitive, and mitochondrial haplotype IIa, which are characteristics found commonly in isolates of P. infestans from potato in Wales (1). Single-sporangial isolates from each isolate were homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100, Pep 100/100). RG57 fingerprint analysis further established that all five black nightshade isolates were identical to each other and to some local P. infestans isolates from potato. P. infestans in Wales belongs to the new population (1), which may infect a wider host-range than the old US-1 clonal lineage. However, infected black nightshade was only found after late blight was widespread in potato fields. In subsequent years at the same site, weeds of S. nigrum have remained noninfected despite high levels of late blight pressure on adjacent potato plots. There is no evidence to suggest that this species acts as an overwintering host in Wales since it is an annual and lacks frost resistance. Field infection of S. nigrum by P. infestans has recently been reported in the Netherlands (2). Our observations confirm the potential of P. infestans to infect another solanaceous plant species. Alternative hosts may interfere with current disease control strategies because infected weeds would escape fungicide application and could serve as reservoirs of inoculum throughout the growing season.

Development of in situ and ex situ seed baiting techniques to detect mycorrhizal fungi from terrestrial orchid habitats

An innovative ex situ fungal baiting method using soil collected from field sites which allows the simultaneous detection of mycorrhizal fungi for multiple terrestrial orchids is presented. This method demonstrated that coarse organic matter (> 2 mm) in the litter and topsoil was the most important reservoir of inoculum of these fungi. A new in situ seed baiting method using multi-chambered packets to simultaneously assess germination for different orchid species within soil is also introduced. These in situ and ex situ methods are compared using seed of orchids in the genera Monadenia, Microtis, Caladenia, Pterostylis and Diuris, using urban Banksia woodland sites with high or low weed cover. Both these seed baiting methods detected compatible fungi for these orchids, but common orchids germinated more frequently than those which were uncommon at the field sites. Germination rates were not significantly affected by weed cover even though adult orchids were rare in areas with high weed cover. The two new seed baiting methods vary in efficiency and applicability depending on the situation where they are used. However, the ex situ method allowed the time-course of germination to be observed, resulting in the production of more protocorms and facilitation of the isolation of mycorrhizal fungi. These techniques provide valuable new tools for detection of compatible mycorrhizal fungi to assist orchid research and conservation.

Occurrence of Leaf Blight on Petunia Caused by Phytophthora infestans in Maryland.

In April 2001, late blight was detected on petunia (Petunia × hybrida Hort.) in a glasshouse in central Maryland. Symptomatic plants had extensive light gray, irregularly shaped, slightly sunken lesions up to 3 cm long on upper leaves and branches. Stems were distorted or dead above points where lesions had coalesced to cause stem girdling. Disease initially occurred as scattered foci but rapidly spread through the crop causing significant loss. The disease reappeared in the same and adjacent greenhouses in 2002. Phytophthora infestans Mont deBary was observed sporulating on leaves and stems of plants in both years. Isolates PE-01, PE-02, and PE-03 were isolated on rye medium from symptomatic stem and leaf tissue (1). To confirm pathogenicity, leaves and stems of 6-wk-old petunia (cv. White Madness) were sprayed with water or with aqueous suspensions of 105 sporangia per ml. Plants were placed in plastic bags in a growth chamber at 20 to 22°C. Bags were removed after 3 days. Five to seven days after spraying, necrotic leaf lesions similar to those on naturally infected plants developed on inoculated plants only. Lesions occurred at leaf margins or near midribs and were 1 to 6 mm, irregular in shape, slightly sunken, grayish, and accompanied by white, fluffy mycelial and sporangial growth. P. infestans was reisolated onto rye agar from lesions from all inoculated plants, completing Koch's postulates. Lesions contained abundant, nonseptate sporangia on obvious sporangiophore structures whose morphology matched that of P. infestans (2). Sporangia from these lesions were 39 to 50 μm (average 45 μm) long × 26 to 28 μm (average 27 μm) wide with a length/breadth ratio of 1.66. Sporangia were caducous and limoniform to ovoid in shape. Oospores were not observed. The fungus grew more slowly on potato dextrose agar (PDA) than on rye medium at 18°C (2.7 versus 9.8 mm per day) and 25°C (1.6 versus 3.0 mm per day). All three isolates mated with an A1 type isolate of P. infestans forming oogonia (36.14 μm in diameter), amphigynous antheridia (22.39 μm × 17.00 μm wide), and aplerotic oospores (16.91 μm in diameter), indicating that the petunia isolates were the A2 mating type. These characteristics were consistent with a heterothallic species such as P. infestans (3). Tomato and potato plants were susceptible to PE-01. The three petunia isolates were US-8 multilocus genotype, resistant to Ridomil and mitochondrial haplotype Ia, characteristics typical of P. infestans from potato in North America. All isolates were monomorphic and homozygous for glucose-6-phosphate isomerase and peptidase (Gpi 100/100; Pep 100/100). Restriction fragment length polymorphism (RG-57) analysis confirmed the close similarity of the petunia and potato isolates. Greenhouse growers who cultivate more than one Solanaceous species should be aware that petunia transplants may have incipient Phytophthora spp. infections serving as reservoirs of inoculum, and that some strains are resistant to metalaxyl-based fungicides. P. infestans has been reported to cause leaf blight on petunia in the northeastern United States (2,4). To our knowledge, this is the first report of natural infection of P. infestans on petunia in Maryland.

Mycotoxigenic Fusarium and deoxynivalenol production repress chitinase gene expression in the biocontrol agent Trichoderma atroviride P1.

Mycotoxin contamination associated with head blight of wheat and other grains caused by Fusarium culmorum and F. graminearum is a chronic threat to crop, human, and animal health throughout the world. One of the most important toxins in terms of human exposure is deoxynivalenol (DON) (formerly called vomitoxin), an inhibitor of protein synthesis with a broad spectrum of toxigenicity against animals. Certain Fusarium toxins have additional antimicrobial activity, and the phytotoxin fusaric acid has recently been shown to modulate fungus-bacterium interactions that affect plant health (Duffy and Défago, Phytopathology 87:1250-1257, 1997). The potential impact of DON on Fusarium competition with other microorganisms has not been described previously. Any competitive advantage conferred by DON would complicate efforts to control Fusarium during its saprophytic growth on crop residues that are left after harvest and constitute the primary inoculum reservoir for outbreaks in subsequent plantings. We examined the effect of the DON mycotoxin on ecological interactions between pathogenic Fusarium and Trichoderma atroviride strain P1, a competitor fungus with biocontrol activity against a wide range of plant diseases. Expression of the Trichoderma chitinase genes, ech42 and nag1, which contribute to biocontrol activity, was monitored in vitro and on crop residues of two maize cultivars by using goxA reporter gene fusions. We found that DON-producing F. culmorum and F. graminearum strains repressed expression of nag1-gox. DON-negative wild-type Fusarium strains and a DON-negative mutant with an insertional disruption in the tricothecene biosynthetic gene, tri5, had no effect on antagonist gene expression. The role of DON as the principal repressor above other pathogen factors was confirmed. Exposure of Trichoderma to synthetic DON or to a non-DON-producing Fusarium mutant resulted in the same level of nag1-gox repression as the level observed with DON-producing FUSARIUM: DON repression was specific for nag1-gox and had no effect, either positive or negative, on expression of another key chitinase gene, ech42. This is the first demonstration that a target pathogen down-regulates genes in a fungal biocontrol agent, and our results provide evidence that mycotoxins have a novel ecological function as factors in Fusarium competitiveness.

Recovery of Pyrenophora tritici-repentis from Barley and Reaction of 12 Cultivars to Five Races and Two Host-Specific Toxins

Barley, a collateral host of Pyrenophora tritici-repentis (cause of tan spot of wheat), could promote genetic variation in the fungal population and serve as a reservoir of local primary inoculum for a wheat crop in the next season. Samples of diseased barley leaves were collected from 35 locations in North Dakota during the 1999 growing season. P. tritici-repentis was recovered from 2 to 5% of the sampled lesions taken from three of the samples. The majority of the samples also harbored Pyrenophora teres, Stagnospora nodorum, and Cochliobolus sativus. All 10 isolates of P. tritici-repentis recovered from the samples were grouped as race 1, because they induced both necrosis and chlorosis on the appropriate wheat differentials. In addition, 12 barley cultivars were tested for their reaction to five races of P. tritici-repentis, and all were resistant. The fungus could not be recovered when isolations were later attempted from the inoculated leaves. Barley cultivars also were infiltrated with two host-specific toxins, Ptr ToxA and ToxB. They were insensitive, because they did not develop necrosis or chlorosis to the toxins. Our results indicate that barley is highly resistant to the fungus and is unaffected by its host-specific toxins. Prevalence of race 1 in a very low frequency on barley during the growing season suggests that barley does not play a significant role in tan spot epidemiology on wheat or promote variation in the fungal population.

Nickel increases susceptibility of a nickel hyperaccumulator to Turnip mosaic virus.

Hyperaccumulated Ni can defend plant tissues against herbivores and pathogens. The effectiveness of this defense, however, has not been tested with a viral pathogen. Turnip mosaic virus (TuMV) accumulation was studied in two serpentine species of Streptanthus with different Ni uptake abilities. Plants of a Ni hyperaccumulator, milk-wort jewelflower (S. polygaloides Gray), and a non-hyperaccumulator, plumed jewelflower (S. insignis Jepson), were grown on Ni-amended and unamended soils. Plants were inoculated with TuMV at three different phenological stages: basal rosette, bolting, and flowering. Susceptibility of experimental plants to TuMV was determined either by the magnitude of TuMV accumulation (measured by indirect enzyme-linked immunosorbent assay [ELISA]) or by plant survival. Streptanthus polygaloides plants grown on high-Ni soil were more susceptible to TuMV than low-Ni S. polygaloides at all three phenological stages. All rosette and pre-bolt S. insignis plants were infected by TuMV, but survival and TuMV accumulation were not significantly affected by soil Ni. At flowering, only high-Ni S. polygaloides plants became infected. For S. polygaloides, elevated tissue Ni concentrations enhanced TuMV infection instead of defending plants from the virus. To reduce risks to nearby agricultural crops, future phytoremed. iation and phytomining operations using this species should incorporate management plans to prevent the creation of artificial reservoirs of TuMV inoculum.

Stagonospora and Septoria diseases of barley, oat, and rye

This paper presents a comprehensive review of the foliar diseases caused by Stagonospora and Septoria fungi on barley, oat, and rye. The relationship of the pathogens to related species that cause disease on wheat is discussed. The diseases are more serious during periods of cool wet weather and are more prevalent in northern Europe and North America. They are frequently associated with the planting of highly susceptible cultivars. The biotype of Stagonospora nodorum that infects barley differs significantly from the biotype that attacks wheat and is occasionally important in the southeastern U.S.A. and northern Europe. Stagonospora avenae f. sp. triticea infects barley, rye, and wheat. It is usually a minor pathogen of barley and rye, but these hosts may serve as an inoculum reservoir for infection of wheat. Stagonospora avenae f. sp. avenaria causes stagonospora leaf blotch and black stem on oat. The black stem phase of the disease causes significant yield reduction and is frequently important in eastern Canada. Morphological characteristics and host range are similar among the Stagonospora species. Teleomorphs of these fungi are in Phaeosphaeria, and the epidemiological importance of this state varies with each pathogen. Septoria passerinii causes speckled leaf blotch on barley. It has become increasingly important in recent years in north central North America and may be associated with reduced tillage. Septoria tritici f. avenae occurs infrequently on oat, and Septoria seculis causes a minor disease on rye. Teleomophic states of these Septoria species have not been identified. Resistance to Stagonospora nodorum has been identified in barley and to Stagonospora avenae f. sp. avenaria and Septoria passerinii in the cultivated hosts and wild relatives.. Race-specific resistance has not been identified and resistance is primarily partial resistance. Recent use of molecular genetics is providing a clearer understanding of the relationships among these and related pathogens on wheat and grasses.

Common anastomosis and internal transcribed spacer RFLP groupings in binucleate Rhizoctonia isolates representing root endophytes of Pinus sylvestris, Ceratorhiza spp. from orchid mycorrhizas and a phytopathogenic anastomosis group

Binucleate Rhizoctonia endophytes from the roots of nursery‐grown Pinus sylvestris (Scots pine) seedlings and the orchid Goodyera repens from Scots pine forests were characterized on the basis of morphological characters, anastomosis group membership and PCR‐assisted ribosomal DNA fingerprinting. Common hyphal and colony morphological traits displayed by the Finnish binucleate Rhizoctonia isolates and a range of Canadian orchid root endophytes enabled them to be placed in the anamorphic genus Ceratorhiza. Five main anastomosis groups were identified and included groups that contained different combinations of Scots pine, G. repens and Canadian Ceratorhiza spp. isolates. Two Scots pine root endophytes anastomosed with a phytopathogenic Japanese tester isolate, confirming their membership of anastomosis group I, which is known to include the teleomorphic species Ceratobasidium cornigerum. Hierarchical cluster analysis of RFLPs in the internal transcribed spacer of ribosomal DNA enabled the division of isolates into one of five RFLP groups. The RFLP and anastomosis groupings were closely correlated; isolates within each of four RFLP groups, which shared 100% RFLP identity, anastomosed in the cross‐pairing anastomosis group tests. However, all represented different vegetatively compatible populations (clones) because the diagnostic killing reaction, a cellular vegetative incompatibility response, was identified at hyphal fusion junctions. These findings indicate a high degree of intraspecific variation within Ceratobasidium cornigerum, which includes isolates able to enter into either mutualistic or pathogenic root association with susceptible host plants. The common anastomosis group/RFLP groupings identified also strongly support the hypothesis that conifer tree roots can act as large inoculum reservoirs for these orchid endophytes, allowing the development of inter‐plant connections, via commonly shared hyphal linkages, in boreal forest ecosystems.

Incidence and Effects of Beet Western Yellows Virus in Western Oregon Sugarbeet Seed Crops

A 1986 to 1995 survey of beet western yellows luteovirus (BWYV) in western Oregon crop and weed species demonstrated that BWYV was endemic in at least 30 plant species. Because sugarbeet-seed crops (Beta vulgaris L.) were sometimes perceived as the primary western Oregon source of BWYV inoculum, we sought to examine all components of the inoculum reservoir. The array of BWYV hosts in western Oregon provided a year-round supply of BWYV inoculum comprising three interactive sub-reservoirs : (a) widespread annual, winter annual, and perennial weeds, (b) BWYV-susceptible vegetable crops, including garden beet, lettuce, spinach, Swiss chard, and turnip, and (c) sugarbeet-seed crops. We defined zones of high and low BWYV incidence in sugarbeet-seed fields within western Oregon, and monitored incidence of BWYV in a common weed species, Brassica campestris L. (field mustard). We demonstrated abrupt variations in BWYV incidence in sugarbeet-seed plantings in the vicinity of Corvallis near the Willamette River. BWYV incidence in sugarbeet-seed fields increased in localities where sugarbeet-seed crops were grown successively. Although marked differences in seed yields were observed, no reduction in sugarbeet seed yield was attributable to BWYV infection.

The indigenous coastal Californian population of the mushroom Agaricus bisporus, a cultivated species, may be at risk of extinction

This study employed nuclear and mitochondrial markers to assess the present‐day composition of the population of Agaricus bisporus in coastal California. Favourable weather in the fall, winter and spring of 1990–91 furnished an uncommon opportunity to collect and study field material of the ‘button mushroom’A. bisporus, a cultivated species, from the region. The previous such season occurred 13 years earlier. Ninety‐five nonredundant cultures from field material were prepared and genotypically characterized. These data were combined with data from earlier studies. Multilocus nuclear and mitochondrial genotypes were determined for 123 individuals. Genotypes were compared in pairwise fashion both within the sample and between this sample and others of diverse geographical origin or commercial provenance. Using parametric analysis and cluster analysis of nuclear similarities, and also mitochondrial data, two elements – indigenous and European – were apparent within the sample. This was consistent with our earlier results on a much smaller sample. At least 10 mitochondrial haplotypes (MTs) were present; based on genotypic similarities of associated nuclei, five (or six) MTs were Californian, four were European, and one was ambiguous. Based on MT origins, 54% of the 121 classifiable individuals in California were of European ancestry; natives constituted a minority at 46%. Even in the indigenous Monterey cypress habitat, where 84% of all individuals from California were sampled, non‐native A. bisporus appeared to have achieved parity (at 48–49%) with the native population. In all other habitats, which are far more extensive, European individuals outnumbered Californian natives by 4:1. Some evidence of hybridization between the two ancestral groups was found. European strains appear to have been resident in California for approximately one century. The extensive occupancy of the native habitat by non‐native germ plasm, the apparent inability of native strains to occupy or compete in non‐native habitats, and the disproportionately large inoculum reservoirs represented by non‐native habitat and agronomic activities all suggest that the native population is under considerable competitive pressure in what may be a very dynamic situation. If this surmise is correct, the native population may be at serious risk of further contraction, irreversible dilution through interbreeding, and possibly even extinction.

Effects of Container Bases on the Spread of Meloidogyne incognita in a Hawaiian Ornamental Nursery.

The influence of container bases on nematode spread from infested pots or ground in an ornamental foliage nursery was investigated with Meloidogyne incognita as the test nematode and cowpea as the indicator plant. The container bases examined were black cloth, concrete masonry blocks, gravel, crushed cinders, and bare ground. Masonry blocks raised 46 cm above ground (raised bench) served as the control treatment. Under the standard nursery practice of irrigation by rainfall and supplemental overhead sprinkler, nematodes spread from contaminated pots or ground to adjacent nematode-free pots on all tested container bases. Incidence of spread, measured as the percentage of nematode-free pots that became contaminated, increased over time and occurred more extensively from ground to pot than from pot to pot. Ground to pot spread increased rapidly following heavy rains, indicating rain was an important contributing factor. On the M. incognita-infested nursery ground, the nematode was found more frequently associated with weeds than with bare soil, suggesting that weeds were important reservoirs of nematode inoculum for rain-splash dispersal. Placement of containers on a raised bench was more effective than on any of the unraised container bases in preventing the ground to pot spread.

Compartment Analysis of Disease Progression in Time. I. Unrestricted Disease Increase

AbstractDisease progression in time through presporulating (latent), sporulating (infectious) and postporulating (removal) stages has been modelled as a three‐compartment system in the case where no upper bound for total disease increase is considered. Deterministic compartment analysis has been performed based on Jeger's linked differential equations. Some important, well‐established epidemilogical principles and concepts have been ‘rediscovered’, clarified or redefined. A new variable, defined as total inoculum reservoir of the host, is suggested as an additional epidemiological concept. Three rate parameters with phytopathological meaning were included in the system: R the constant intrinsic infection rate comparable to the Vanderplankian R,c, h pathogen's ‘hatching rate’, related to the mean latent period p and g the intrinsic removal rate related to the infectious period i. Another parameter λ1 is empirical and comparable to the logarithmic, infection rate, sensu Vanderplank if time becomes very large. In the compartment system iR= 0 is a threshold condition for total disease to increase over initial disease level and iR= 1 is a threshold condition for an ‘explosive’ total disease increase that may induce a large epidemic rather than a threshold for total disease to start or to exist. When iR < 1, the total disease level approaches α asymptotically, a finite value, while when iR > 1, the total disease, increase is explosive; only where λ1=√dh does this increase follows the exponential law at larger t, otherwise the exponential law with the same rate parameter λ1 underestimates or overstimates the amount of total disease at any time t. Some further implications are discussed and compared with those derived from the Vanderplankian system of epidemiological analysis.

Compartment Analysis of Disease Progression in Time. II. Restricted Disease Increase

AbstractThree‐compartmenl analysis was performed to describe the disease motion through presporulating (latent), sporulating (infectious) and postsporulating (removal) stages in the general case, where total disease increase is density dependent. A second‐order Runge‐Kutta method for numerical integration of the system of difterential equations was used to solve the equations. Total and postsporulating disease progress curves are S‐shaped while latent and infectious disease progress in the form of optimum curves. The curve of a composite variable defined as total (latent and infectious) inoculum reservoir of the host progresses also in the form of an optimum curve showing a maxi at the total disease level yt/K = 1 − 1/iR, where K is the total population of infection sites, i is the infectious period, R the intrinsic infection rate and iR ≥ 1/(1 − Vo/K). The size of this maxi is a monotone increasing function of the product iR by given initial disease level Vo. Condition iR =1/(1−yo/K) is a threshold condition for total inoculum to increase over yo, or alternatively a threshold condition for a rapid disease increase at the start resulting, possibly, in a large epidemic. Condition iR = O is a threshold condition for total disease to increase over initial disease level. Total disease reaches an asymptotic value less than unity if and only if infectious period is linite (existence of removals). In the compartment system there is a consistency regarding the threshold conditions for total disease to increase over initial disease level in the cases with and without a density‐dependent factor. Conversely, in the Vanderplankian system of differential‐difference equation the threshold conditions are iR = 0 and iR = 1 respectively due to the assumption of an exponential increase of total disease early in the epidemic. The particular cases without latent period and without removals are treated separately. The implications derived from the compartment analysis are compared with those derived from the Vanderplankian system of epidemiological analysis.

Field persistence of Bradyrhizobium sp. (Lupinus) inoculant for serradella (Ornithopus compressus L.)

Experiments were conducted, in situ in north-western New South Wales and in the laboratory, to evaluate the longevity of Bradyrhizobium sp. (Lupinus) inoculated as a peat culture onto seed of yellow serradella (Ornithopus compressus L.). Observations were made on the field persistence of the inoculant strain over 2 years. Two categories of seed were examined: seed encased in a segment of pod coat (hulled), and seed from which the pod coat had been removed (dehulled). The use of adhesive (methyl cellulose or gum arabic) ensured that the inoculant adhered to the seed. At a constant temperature of l8�C, inoculant applied to hulled seed survived for >8 months without loss of viability. Type of adhesive did not affect survival. At the 2 field sites, inoculated seed was sown into dry soil. Inoculant was sufficiently viable for 77-159 days to colonise the rhizospheres of young serradella and to form nodules on a significant (P<0.05) proportion of the plants. Rhizosphere colonisation and nodulation were better with hulled seed than with dehulled seed but were not affected by type of adhesive. Evidence is presented that the inoculant was able to establish in these soils as a permanent component of the soil microflora and to persist as a reservoir of inoculum for annually regenerating serradella.

Acidovorax avenae subsp. avenae . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Acidovorax avenae subsp. avenae . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Agropyron intermedium, A. trichophorum, Avena sativa, Bromus catharticus, B. marginatus, Camellia sinensis (may form a complex with Pseudomonas syringae pv. theae : 68, 955), Caryota mitis, Digitaria sanguinalis, Echinochloa crusgalli, Eleusine coracana, Euchlaena mexicana, Oryza sativa, Panicum hirsutum, P. miliaceum, Paspalum nutans, P. paniculatum, P. urvillei, Pennisetum americanum, Saccharum officinarum, Setaria italica, S. lutescens, S. viridis, Sorghum bicolor and Zea mays . DISEASE: Bacterial leaf blight of maize and sorghum, brown stripe of rice and red stripe of sugarcane. Symptoms in general consist of leaf streaks and stripes, which may extend into the sheaths. Occasionally a stalk rot develops. Symptoms are more severe on seedlings and immature plants. Discolouration of seeds can occur on rice. In fishtail palm ( Caryota mitis ), lesions are mainly at the leaf margin and brown/black with a chlorotic halo (Miller, 1992). In most cases conditions of high temperature and high relative humidity favour symptom development. GEOGRAPHICAL DISTRIBUTION: Comoro Is., Egypt, Ethiopia, Ivory Coast, Kenya, Madagascar, Mauritius, Malawi, Mozambique, Niger, Nigeria, Reunion, Sierra Leone, South Africa, Sudan, Tanzania, Uganda, Zaire, Zimbabwe, Bangladesh, Burma, Bhutan, Cambodia, China, India, Indonesia, Iran, Iraq, Japan, Korea, Malaysia, Nepal, Okinawa, Pakistan, Philippines, Sri Lanka, Taiwan, Thailand, Vietnam, Australia, Fiji, Guam, Papua New Guinea, Tahiti, Italy, Portugal, Turkey, Mexico, USA (Alabama, Arkansas, Florida, Georgia, Illinois, Hawaii, Kansas, Louisiana, MT, Nebraska, New York, Pennsylvania, Texas & Virginia), Barbados, Costa Rica, Cuba, Dominican Republic, El Salvador, Guadeloupe, Guatemala, Honduras, Jamaica, Martinique, Nicaragua, Panama, Puerto Rico, St. Kitts, Trinidad, Argentina, Brazil (Sao Paulo), Colombia, Guyana, Paraguay, Peru, Surinam, Uruguay, Venezuela ( CMI Map 511 ed. 1, 1976 & CMI Map 39 ed. 5, 1987). TRANSMISSION: The disease is largely thought to be seed borne. In rice, the bacterium is located between the glumes and the pericarp, or deeper in the seed (Shakya et al. , 1986). There is evidence to suggest that mature plants which survive infection in the seedling stage harbour latent infections. The bacterium can be found in 8-yr old rice seed samples stored at 5°C (Shakya et al. , 1985). The bacterium is not thought to survive well in soil or in plant debris. Alternative hosts such as Paspalum urvillei (Vasey grass) in Florida, USA have been noted as an inoculum reservoir in bacterial leaf blight of maize outbreaks (Shurtleff, 1973; 57, 4929).

A new species of gall midge (Diptera: Cecidomyiidae) attacking mango foliage in Guam, with observations on its pest status and biology

AbstractA new species of gall midge,Procontarinia schreineriHarris, which attacks mango foliage in Guam, is described and the results of field observations on its pest status, biology and population fluctuations are reported. Eggs are laid on young mango leaves and larvae, which develop rapidly over about 5 days, induce blister galls before leaving to pupate. Secondary damage to infested foliage is caused by the fungusColletotrichum gloeosporioides, which invades damaged leaf tissue and causes mango anthracnose disease.P. schreineripopulation fluctuations on mangoes were assessed at four localities on Guam over three years by counting the numbers of galls. It was concluded that the main factors affecting populations were rainfall and location. More galls were present during rainy periods, possibly because high humidity improves larval and pupal survival. Gall populations were generally low and unlikely to have direct effects on fruit yields but the introduction of new improved varieties of mango might increase susceptibility to damage. In addition, old galls on damaged leaves may provide reservoirs of anthracnose inoculum.

Occurrence of a picorna-like virus in planthopper species and its transmission in Laodelphax striatellus

Himetobi P virus (HiPV) was detected in three planthopper species from the temperate monsson region of Japan. High incidences of infection occurred in some cultures of Laodelphax striatellus (range: 0–100%), Nilaparvata lugens (0–88%), and Sogatella furcifera (0–58%). In the field HiPV was only detected, at low incidence, in L. striatellus (0–6%). The virus was not detected in three other homopterans: Nephotettix cincticeps, Rhopalosiphum maidis, and R. padi, which feed on the Poaceae. High concentrations of HiPV were detected in adults and nymphs of L. striatellus. The virus was not detected in eggs and was not transovarially transmitted. Maternal contamination and contamination from surrounding plant tissues may be a source of inoculum for hatching L. striatellus eggs. Frass contained high concentrations of HiPV and is a likely source of inoculum. HiPV did not multiply in rice or wheat nor was it translocated from inoculated leaves to other parts of the plant. Therefore host plants form, at best, a transient reservoir of inoculum. HiPV transmission is compared with that of rice stripe tenuivirus which infects both L. striatellus and its plant hosts rice and wheat.

Colonization of Strawberry Roots by VA Mycorrhizal Fungi in Agroecosystems under Conventional and Transitional Organic Management

ABSTRACT Mycorrhizal colonization of strawberry roots was studied for two agricultural management systems on the California central coast, one conventional and one in transition to organic. Low levels of VAM colonization early in the first year of the experiment may be the result of past management practices, which included conventional production of a non-mycorrhizal crop with high levels of phosphorus inputs. In the second year colonization levels were higher in the transitional organic plots. Annual use of a fumigant and a fungicide in the conventional plots was implicated with depleting the VAM inoculum reservoir and maintaining low colonization levels. Inoculation of organically managed plots with Glomus intraradices at the second planting did not change colonization levels. Soil analyses showed that available soil phosphorus levels remained high (range 62 to 101 ppm) in both treatments throughout the study. In both years colonization increased during the spring and summer harvest season, and it is h...

Health Status (Virus) of Native North American Humulus lupulus in the Natural Habitat

AbstractNinety‐one native North American Humulus lupulus plants from natural habitats in seven western and mid‐western states of the U.S.A. were tested by ELISA serology for presence of two ilarviruses and three carlaviruses common to cultivated hops. All plants in natural habitats were free of detectable viruses. Propagations of 14 such plants from earlier collections had become infected, particularly with two carlaviruses (hop latent virus, American hop latent virus) after exposure for 22 years tobreeding nurseries. ELISA tests of some 284 hop plants primarily from breeding nurseries in Oregon indicated the following infection rates: Prunus necrotic ringspot virus, 85/284, 30%; apple mosaic virus, 88/234, 38%; hop mosaic virus, 59/158, 37 %; hoplatent virus, 104/158, 66 %; and American hop latent virus, 79/158, 50 %. Inoculum reservoirs of AHLV were sought among 53 principally perennial non‐Humulus plant species surrounding AHLV‐infected hop yards and nurseries. AHLV was neither indigenous to native North American H. lupulus nor detectable in these selected non‐Humulus plant species. Breeding nurseries and commercial hop yards, thus, were the only detectable inoculum reservoir for AHLV.

Observations on the prevalence and incidence of ergot disease in Great Britain with special reference to open‐flowering male‐sterile cereals

SUMMARYErgot (Claviceps purpurea) of grasses is prevalent throughout Britain. The disease also occurs regularly in most cereal varieties but generally is of low incidence. Incidence is much greater in open‐flowering and male‐sterile cereals, with the infection seeming to come, initially, from the large reservoir of inoculum amongst grasses. The production of F1‐hybrid cereal seed, which necessitates the use of male‐sterile lines, could therefore be jeopardised.