Foliar Plant Pathogens Research Articles

Differential tolerance of Zymoseptoria tritici to altered optimal moisture conditions during the early stages of wheat infection

Foliar plant pathogens require liquid or vapour water for at least part of their development, but their response and their adaptive tolerance to moisture conditions have been much less studied than other meteorological factors to date. We examined the impact of altering optimal moisture conditions conducive to infection on the wheat-Zymoseptoria tritici interaction. We assessed the responses in planta of 48 Z. tritici strains collected in two climatologically distinct locations (Ireland and Israel) to four high moisture regimes differing in the timing and the duration of uninterrupted exposure to saturated relative humidity (100% RH) during the first three days of infection. Individual- and population-level moisture reaction norms expressing how the sporulating area of a lesion change with the RH conditions were established based on visual assessments of lesion development at 14, 17 and 20 days post-inoculation (dpi). Our findings highlighted: (i) a critical time-dependent effect on lesion development of uninterrupted periods of exposure to 100% RH during these earliest infection stages; (ii) a marked interindividual variation in the sensitivity to RH conditions both in terms of strain average moisture response and plasticity; (iii) a higher tolerance – expressed at 14 dpi, not later – of the Israeli population to early interruption of optimal moisture conditions. By indicating that sensitivity to sub-optimal moisture conditions may vary between individuals and populations, this study highlights the evidence of moisture adaptation signature in a plant pathogen. Understanding such variation is helpful to predict the response of pathogen populations to changing climatic conditions.

Fire Ant Venom Alkaloids: Possible Control Measure for Soilborne and Foliar Plant Pathogens.

The purpose of this study was to evaluate fire ant venom alkaloids and an alarm pheromone analog against several plant pathogens, including Botrytis cinerea, Fusarium oxysporum, Phytophthora nicotianae, P. cryptogea, Pseudomonas syringae, Phytopythium citrinum, Rhizoctonia solani, Sclerotonia rolfsii, Xanthomonas axonopodis, and X. campestris. All pathogens were tested against red imported fire ant venom alkaloid extract and alarm pheromone compound for growth inhibition in in vitro assay. The venom alkaloid extract inhibited fungal and oomycete pathogens. Neither of the treatments were effective against bacterial pathogens. Three soilborne pathogens, P. nicotianae, R. solani, F. oxysporum, and one foliar pathogen, B. cinerea were selected for further in-vivo assays on impatiens (Impatiens walleriana ‘Super Elfin XP violet’). Total plant and root weight were higher in venom alkaloid treated plants compared to an inoculated control. The venom alkaloid treatment reduced damping-off, root rot severity, and pathogen recovery in soilborne pathogen inoculated plants. Similarly, venom alkaloid reduced Botrytis blight. However, higher venom rates caused foliar phytotoxicity on plants. Therefore, additional work is needed to evaluate rates of venom alkaloids or formulations to eliminate negative impacts on plants. Overall, these results suggest that red imported fire ant venom alkaloids may provide a basis for new products to control soilborne and foliar plant pathogens.

Antifungal Activity of Stabilized Ortho Silicic Acid (OSA) against Foliar Plant Pathogens

Present study was aimed to evaluate antifungal activity of stabilized ortho silicic acid (OSA) against foliar pathogens under in vitro and in vivo conditions. Studies were conducted using a commercial formulation of stabilized ortho silicic acid (2.0%, Silixol™ OSA). Activity of OSA was evaluated on thirteen fungal pathogens (Alternaria alternata, Alternaria macrospora, Alternaria solani, Cercospora arachidicola, Colletotrichum gloeosporioides, Corynespora cassiicola, Curvularia lunata, Bipolaris sorokiniana, Mycosphaerella fragariae, Mycopshaerella musicola, Phoma obscurans, Phyllosticta arachidis-hypogaeae, and Pyricularia oryzae) using Poison Food Technique. Silixol OSA exhibited a differential activity against all test pathogens. Out of thirteen pathogens, eight exhibited more than 50% inhibition of mycelial growth. Silixol OSA had induced ultrastructure changes in both fungal mycelium and spores. Pathogens with more than 50% inhibition, based on Poison Food Technique, were tested further by in vivo studies. OSA sprayed plants had healthy leaves and exhibited less disease severity. Results of the present study elucidate potential application of stabilized OSA in Integrated Disease Management Program (IDMP) to avoid reliance on synthetic fungicides.

Pseudomonas syringae AlgU Downregulates Flagellin Gene Expression, Helping Evade Plant Immunity.

Flagella power bacterial movement through liquids and over surfaces to access or avoid certain environmental conditions, ultimately increasing a cell's probability of survival and reproduction. In some cases, flagella and chemotaxis are key virulence factors enabling pathogens to gain entry and attach to suitable host tissues. However, flagella are not always beneficial; both plant and animal immune systems have evolved receptors to sense the proteins that make up flagellar filaments as signatures of bacterial infection. Microbes poorly adapted to avoid or counteract these immune functions are unlikely to be successful in host environments, and this selective pressure has driven the evolution of diverse and often redundant pathogen compensatory mechanisms. We tested the role of AlgU, the Pseudomonas extracytoplasmic function sigma factor σE/σ22 ortholog, in regulating flagellar expression in the context of Pseudomonas syringae-plant interactions. We found that AlgU is necessary for downregulating bacterial flagellin expression in planta and that this results in a corresponding reduction in plant immune elicitation. This AlgU-dependent regulation of flagellin gene expression is beneficial to bacterial growth in the course of plant infection, and eliminating the plant's ability to detect flagellin makes this AlgU-dependent function irrelevant for bacteria growing in the apoplast. Together, these results add support to an emerging model in which P. syringae AlgU functions at a key control point that serves to optimize the expression of bacterial functions during host interactions, including minimizing the expression of immune elicitors and concomitantly upregulating beneficial virulence functions.IMPORTANCE Foliar plant pathogens, like Pseudomonas syringae, adjust their physiology and behavior to facilitate host colonization and disease, but the full extent of these adaptations is not known. Plant immune systems are triggered by bacterial molecules, such as the proteins that make up flagellar filaments. In this study, we found that during plant infection, AlgU, a gene expression regulator that is responsive to external stimuli, downregulates expression of fliC, which encodes the flagellin protein, a strong elicitor of plant immune systems. This change in gene expression and resultant change in behavior correlate with reduced plant immune activation and improved P. syringae plant colonization. The results of this study demonstrate the proximate and ultimate causes of flagellar regulation in a plant-pathogen interaction.

Comparative genomics of Pseudomonas syringae pathovar tomato reveals novel chemotaxis pathways associated with motility and plant pathogenicity.

The majority of bacterial foliar plant pathogens must invade the apoplast of host plants through points of ingress, such as stomata or wounds, to replicate to high population density and cause disease. How pathogens navigate plant surfaces to locate invasion sites remains poorly understood. Many bacteria use chemical-directed regulation of flagellar rotation, a process known as chemotaxis, to move towards favorable environmental conditions. Chemotactic sensing of the plant surface is a potential mechanism through which foliar plant pathogens home in on wounds or stomata, but chemotactic systems in foliar plant pathogens are not well characterized. Comparative genomics of the plant pathogen Pseudomonas syringae pathovar tomato (Pto) implicated annotated chemotaxis genes in the recent adaptations of one Pto lineage. We therefore characterized the chemosensory system of Pto. The Pto genome contains two primary chemotaxis gene clusters, che1 and che2. The che2 cluster is flanked by flagellar biosynthesis genes and similar to the canonical chemotaxis gene clusters of other bacteria based on sequence and synteny. Disruption of the primary phosphorelay kinase gene of the che2 cluster, cheA2, eliminated all swimming and surface motility at 21 °C but not 28 °C for Pto. The che1 cluster is located next to Type IV pili biosynthesis genes but disruption of cheA1 has no observable effect on twitching motility for Pto. Disruption of cheA2 also alters in planta fitness of the pathogen with strains lacking functional cheA2 being less fit in host plants but more fit in a non-host interaction.

Eggplant and related species are promising genetic resources to dissect the plant immune response to Pseudomonas syringae and Xanthomonas euvesicatoria and to identify new resistance determinants.

The apparent lack of durability of many resistance (R) genes highlights the need for the constant identification of new genetic sources of resistance for the breeding of new disease-resistant crop cultivars. To this end, we screened a collection of accessions of eggplant and close relatives for resistance against Pseudomonas syringae pv. tomato (Pto) and Xanthomonas euvesicatoria (Xeu), foliar plant pathogens of many solanaceous crops. Both pathogens caused substantial disease on most genotypes of eggplant and its relatives. Promisingly, however, some of the genotypes were fully or partially resistant to either of the pathogens, suggesting the presence of effective resistance determinants in these genotypes. Segregation of resistance to the growth of Xeu following infiltration in F2 progeny from a cross of a resistant and susceptible genotype suggests that resistance to Xeu is inherited as a multigenic trait. With regard to Pto, a mutant strain lacking all 28 functional type III secreted effectors, and a Pseudomonas fluorescens strain expressing a P. syringae type III secretion system (T3SS), both elicit a strong cell death response on most eggplant lines. Several genotypes thus appear to harbour a mechanism for the direct recognition of a component of the T3SS. Therefore, eggplant and its close relatives are promising resources to unravel novel aspects of plant immunity and to identify new candidate R genes that could be employed in other Solanaceae in which Xeu and Pto cause agriculturally relevant diseases.

Numerical Studies of Biocontrol Efficacies of Foliar Plant Pathogens in Relation to the Characteristics of a Biocontrol Agent

A previously published generic mathematic model has been used in a numerical study to understand the dynamics of foliar pathogens in relation to mechanisms, and timing and coverage of biocontrol agent (BCA) applications. With the model parameter values used, it was demonstrated that a BCA possessing either competition or induced resistance as the main mechanism of biological control was more effective in reducing disease development than a BCA with either mycoparasitism or antibiosis as its mechanism. Application coverage, ranging from 50 to 90%, had little effect on biocontrol efficacy, particularly for a BCA with competition and induced resistance as the main mechanism of biocontrol. Conversely, delayed application of BCA had more profound effects on biocontrol efficacy for those with competition or induced resistance as their main mechanism than those with mycoparasitism and antibiosis. Biocontrol efficacy was greatest for a single BCA combining competition with mycoparasitism or antibiosis. The efficacy for a single BCA combining induced resistance with competition critically depended on application time; the efficacy was greatly reduced for delayed applications. The present study suggests that development of an effective strategy for BCA application is critically dependent upon our quantitative understanding of several key biocontrol processes and their interactions. Without reliable quantitative estimation of these processes, it is impossible to make quantitative predictions about biological control and hence to optimize BCA application strategies.

Soil moisture mediated interaction between Polygonatum biflorum and leaf spot disease

Fungal pathogens can regulate the abundance and distribution of natural plant populations by inhibiting the growth, survival, and reproduction of their hosts. The abiotic environment is a crucial component in host–pathogen interactions in natural plant populations as favorable conditions drive pathogen development, reproduction, and persistence. Foliar plant pathogens, such as fungal lesions referred to generically as “leaf spot disease,” are particularly responsive to increased moisture levels, but the manner in which the abiotic environment drives disease dynamics, and how these diseases regulate natural plant populations, is not fully understood. We investigate (1) the impact of ambient soil moisture and diffuse light on the prevalence of a leaf spot pathogen (Phyllosticta sp.) in a natural population of Polygonatum biflorum, an understory herb native to deciduous forest understories in the eastern US, and (2) the effects of the fungal pathogen on the survival, growth, and abundance of the plants. We tracked six P. biflorum populations and disease incidence, as well as soil moisture and diffuse light, between 2003 and 2005 in the understory deciduous forest of the southern Appalachian Mountains, North Carolina, USA. Results show that both the occurrence of P. biflorum and the prevalence of P. biflorum leaf spot disease are highest where soil moisture is intermediate and diffuse light is lowest. Disease occurrence depends upon plant presence, but it also adversely impacts plant survival, abundance, and growth. These results suggest that leaf spot disease is likely to impact population dynamics, which in turn vary as a function of environmental drivers.

Suppression of the bacterial spot pathogen Xanthomonas euvesicatoria on tomato leaves by an attenuated mutant of Xanthomonas perforans.

A bacteriocin-producing strain of the bacterial spot of tomato plant pathogen, Xanthomonas perforans, with attenuated pathogenicity was deployed for biocontrol of a bacteriocin-sensitive strain of the genetically closely related bacterial spot of tomato plant pathogen, X. euvesicatoria. The attenuated mutant (91-118DeltaopgHDeltabcnB) of X. perforans was tested in leaf tissue and shown to significantly inhibit internal populations of the wild-type X. euvesicatoria strain although significantly less than the wild-type 91-118 strain, whereas in a phyllosphere inhibition assay, the mutant strain reduced epiphytic populations comparably to 91-118. Thus, the attenuated mutant limited the sensitive bacterium more efficiently on the leaf surface than inside the leaf. In field experiments, weekly application of 91-118DeltaopgHDeltabcnB significantly reduced X. euvesicatoria populations compared to the growers' standard control (copper hydroxide and mancozeb applied weekly and acibenzolar-S-methyl applied every 2 weeks). The biological control agent, 91-118DeltaopgHDeltabcnB, applied every 2 weeks also significantly reduced X. euvesicatoria populations in one season but was not significantly different from the growers' standard control. Potentially, attenuated pathogenic strains could be deployed as biological control agents in order to improve disease control of foliar plant pathogens.

Control of the bean rust fungus Uromyces appendiculatus by means of Trichoderma harzianum: leaf disc assays on the antibiotic effect of spore suspensions and culture filtrates

Several species of the fungal genus Trichoderma act as antagonists of other fungi. A number of strains from the Trichoderma species T. harzianum Rifai are used as biological control agents for the control of soilborne as well as foliar plant pathogens. Six T. harzianum strains, five of them isolated from commercial preparations, were evaluated for their capability to control the bean rust fungus Uromyces appendiculatus (Pers. ex Pers.) Unger. Different kinds of leaf disc assays were performed with conidial spore suspensions and sterile culture filtrates of the T. harzianum strains. Great differences were observed concerning the efficacy of the Trichoderma strains to reduce the number of the uredial pustules developing after rust inoculation which followed the application of the particular Trichoderma strains. Efficacy values ranged from 1 to over 50%. Increasing spore or culture filtrate concentrations of the two most effective isolates T12 and TU led to decreases in the number of developing uredial pustules. Culture filtrate applications had a protective but no curative effect. T12 spore suspensions maintained their disease reducing activity even when autoclaved. This and some other evidence for an antibiotic interaction between T. harzianum and U. appendiculatus are discussed.

Reduction of bacterial leaf spot severity on radish, lettuce, and tomato plants grown in compost-amended potting mixes

Compost-amended substrates offer the potential for management of diseases caused by soilborne as well as foliar plant pathogens. In this study, the efficacy of composted pine bark mix fortified with the biocontrol agent Trichoderma hamatum 382 (FCPB) against bacterial leaf spot of radish, lettuce, and tomato under controlled environment conditions was evaluated. Plants grown in the FCPB mix and inoculated with bacterial leaf spot pathogens were less severely diseased than plants grown in commercial peat mix or vermiculite. In some cases, plants were also grown in a composted cow manure mix or in a steam-treated compost-amended greenhouse soil, and these plants were also less severely diseased. Infected radish and tomato plants grown in these compost-amended substrates also harbored significantly smaller populations of Xanthomonas campestris pv. armoraciae and Xanthomonas campestris pv. vesicatoria, respectively. The disease-suppression effect of the FCPB mix was lost by autoclaving and restored by reinoculating the autoclaved FCPB mix with nonautoclaved FCPB mix. However, the disease-suppression effect varied among the batches of FCPB mix used. In contrast, vermiculite and a highly decomposed sphagnum peat mix consistently failed to suppress these diseases. These results suggest that producing vegetable seedlings in FCPB mix or compost-based substrates may provide initial protection against bacterial leaf spot pathogens.

Compost Tea: Principles and Prospects For Plant Disease Control

An increasing body of experimental evidence indicates that plant disease can be suppressed by treating plant surfaces with a variety of water-based compost preparations, referred to in the literature as watery fermented compost extracts or compost teas. The terms nonaerated compost teas (NCT) and aerated compost teas (ACT) are used in this review to refer to the common production methods that diverge in the intent to actively aerate. Very little data directly compares the efficacy of NCT and ACT for plant disease suppression. A variety of foliar plant pathogens and/or diseases have been suppressed by applications of NCT while few controlled studies have examined ACT. For some diseases the level of control would be considered inadequate for conventional agriculture; organic producers with limited control options consider partial disease control to be an important improvement. For both compost tea production methods, decisions that influence pathogen suppression include choice of compost feedstocks, compost age, water ratio, fermentation time, added nutrients, temperature and pH. Application technology choices include the dilution ratio, application equipment, timing, rates, spray adjuncts and adding specific microbial antagonists. Increased understanding of compost tea microbiology and the survival and interactions of microbes on plants surfaces should make it possible to modify compost tea production practices and application technology to optimize delivery of a microflora with multiple modes of pathogen suppression. Innovative growers and practitioners are leading the development of new compost tea production methods and uses, generating many potential research opportunities. The use of compost tea as part of an integrated plant health management strategy will require much additional whole systems research by a cohesive team of farmers and experts in composting, plant pathology, phyllosphere biology, molecular microbial ecology, fermentation science, plant physiology, plant breeding, soil science, and horticulture.

A MAP kinase of the vascular wilt fungus Fusarium oxysporum is essential for root penetration and pathogenesis

The soil-borne vascular wilt fungus Fusarium oxysporum infects a wide variety of plant species by directly penetrating roots, invading the cortex and colonizing the vascular tissue. We have identified fmk1, encoding a mitogen-activated protein kinase (MAPK) of F. oxysporum that belongs to the yeast and fungal extracellular signal-regulated kinase (YERK1) subfamily. Targeted mutants of F. oxysporum f. sp. lycopersici carrying an inactivated copy of fmk1 have lost pathogenicity on tomato plants but show normal vegetative growth and conidiation in culture. Colonies of the fmk1 mutants are easily wettable, and hyphae are impaired in breaching the liquid-air interface, suggesting defects in surface hydrophobicity. Fmk1 mutants also show reduced invasive growth on tomato fruit tissue and drastically reduced transcript levels of pl1 encoding the cell wall-degrading enzyme pectate lyase. Conidia of the mutants germinating in the tomato rhizosphere fail to differentiate penetration hyphae, resulting in greatly impaired root attachment. The orthologous MAPK gene Pmk1 from the rice leaf pathogen Magnaporthe grisea complements invasive growth and partially restores surface hydrophobicity, root attachment and pathogenicity in an fmk1 mutant. These results demonstrate that FMK1 controls several key steps in the pathogenesis of F. oxysporum and suggest a fundamentally conserved role for the corresponding MAPK pathway in soil-borne and foliar plant pathogens.

On the velocity of epidemic waves in model plant disease epidemics

Previous work on the temporal and spatial spread of plant diseases has focused on models generating epidemics with constant frontal velocity (traveling epidemic waves). But the theory of turbulent diffusion, the laws of which govern the dispersal of windborne spores of most foliar plant pathogens, suggests that spread becomes more efficient as the area with diseased plants expands. This should result in epidemics with increasing frontal velocity (dispersive epidemic waves). As a first step toward an analytic treatment of dispersive waves of plant diseases this study considers two simple models based on reaction-diffusion equations with scale-dependent diffusivity. Predictions from these models are compared qualitatively to data on the spread of potato late blight ( Phytophthora infestans) epidemics over different temporal and spatial scales. The paper concludes with a brief discussion on the merit of simple, analytical models for studying, interpreting, and testing of abstract epidemiological concepts, such as traveling and dispersive waves.

Glucanolytic enzyme production by Schizophyllum commune Fr. during mycoparasitism

Schizophyllum commune S-25 was able to attack 16 out of 50 fungi tested, representing oomycetes, zygomycetes and hyphomycetes, which are either saprophytic, soilborne or foliar plant pathogens. In dual culture agar plates consisting of S. commune and its host Rhizoctonia solani, the production of extracellular endo-β-1,3(4)-glucanase was markedly enhanced, whereas chitinase, N-acetylglucosaminidase and glucosidase were not or barely detected. Coincidentally, the major constituents of the crude enzyme from dual culture broth was endo-β-1,3(4)-glucanase. Treatment of the young hyphae of R. solani by these crude enzyme preparations led to the release of protoplasts. The specific activity of the endo-β-1,3(4)-glucanase from crude preparations was increased 240 times by DEAE-Sephadex ion exchange chromatography, Sephadex G-100 get filtration and isoelectric focusing. The purified endo-β-1,3(4)-glucanase was capable of hydrolysing purified cell walls of R. solani. It is suggested that endo-β-1,3(4)-glucanase in cooperation with trace amounts of certain wall-lytic enzymes, such as cellulase produced by S. commune may play a crucial role in host parasite interactions.

Fungal foliar plant pathogen epidemics: modelling and qualitative analysis

A new approach for modelling epidemics caused by fungal foliar plant pathogens is presented. The model is described by a system of differential equations, and is simple enough to allow a qualitative analytical study of the main epidemiological factors that determine the rate of disease progress during a single growing period. The model was analysed and compared with Vanderplank's basic equations of plant disease development and some of Jeger's models. Results of qualitative investigation of the model, and close similarity between simulated and observed dynamics of disease development, show that this new approach may be used to compare and analyse some foliar fungal disease epidemics in more detail than formerly.

Wireworm Control on Potato, 1989

Abstract Certified ‘La Rouge’ seed pieces were planted on 16 Dee in a Perrine marl soil and spaced 10 cm apart in rows that were 0.9 m apart. Sixteen treatments were arranged into a randomized complete block design with 4 replications. Treatment plots were 4 rows by 9.1 m long. Plants were fertilized with 896 kg/ha of granular (8:20:8) at planting and an additional 1456 kg/ha of liquid (4:4:12) during the growing season. Maneb 80 was applied (1.7 kg [AI]/ha) weekly and Benlate 50 was applied (1.1 kg [AI]/ha) on two dates to control foliar plant pathogens. Insecticide treatments were applied on three dates. Parathion was applied with a 9.1 m long Gandy applicator and incorporated into the soil with a disc harrow on 16 Dec. Thimet 20 G (4.0 kg [AI]/ha) was applied directly to the seed pieces in the furrow at planting on 16 Dec with a planter-mounted Gandy applicator. Thimet 20 G (2.0 kg [AI]/ha) and Force 1.5 G (0.1 and 0.3 kg [AI]/ha) were applied in the furrows 3 days after planting. Post-emergence applications of Thimet 20 G were also made 8 inches off center to both sides of each hill using a tractor-mounted, chain-driven Gandy and incorporated into the soil with a rolling cultivator after plant emergence on 17 Jan. Post-emergence applications of Vydate and Disyston 8 were made on 19 Jan using fumigator knives that delivered 1402 liter/ha at 4.8 km/h directly to the root zone 8 inches off center on both sides of each hill. Vines were killed on 13 Mar with Paraquat Plus (2.3 liter/ha; 29.1% [AI]). Two 6.1 m sections of row per treatment plot were harvested on 19 Apr. Tubers were washed, graded for wireworm injury (holes less than 1.9 cm in depth), damage (holes greater than 1.9 cm in depth), cracking, sunburn, disease, etc., and weighed fresh. Data for injury, damage, sunburn, and cracking are presented at a percentage of total fresh weight/section of row.

Epidemics of plant diseases: Mathematical analysis and modeling

Previous work on the temporal and spatial spread of plant diseases has focused on models generating epidemics with constant frontal velocity (traveling epidemic waves). But the theory of turbulent diffusion, the laws of which govern the dispersal of windborne spores of most foliar plant pathogens, suggests that spread becomes more efficient as the area with diseased plants expands. This should result in epidemics with increasing frontal velocity (dispersive epidemic waves). As a first step toward an analytic treatment of dispersive waves of plant diseases this study considers two simple models based on reaction-diffusion equations with scale-dependent diffusivity. Predictions from these models are compared qualitatively to data on the spread of potato late blight (Phytophthora infestans) epidemics over different temporal and spatial scales. The paper concludes with a brief discussion on the merit of simple, analytical models for studying, interpreting, and testing of abstract epidemiological concepts, such as traveling and dispersive waves.

Wireworm Control on Potato, 1987

Abstract Certified 'La Rouge' seed pieces were planted on 10 Dec in a Perrine marl soil and spaced 4 inches apart in rows that were 3 ft apart. Thirteen treatments were arranged into a randomized complete block design with 4 replications. Treatment plots were 4 rows by 40 ft long. Plants were fertilized with 800 lb/acre of granular fertilizer (8:20:8) at planting and an additional 1300 lb/acre of liquid fertilizer (4:4:12) during the growing season. Maneb was applied (1.5 lb [AI]/acre) weekly and Benlate 50WP was applied (1.0 lb [AI]/acre) on 2 dates to control foliar plant pathogens. Insecticide treatments were applied on 4 dates. Dyfonate was applied with a 10-ft-long Gandy applicator and incorporated into the soil with a disk harrow on 9 Dec. Thimet was applied directly to the seed pieces in the furrow at planting with a planter-mounted Gandy applicator. Postemergence applications of Vydate and Di-Syston were made on 17 Jan using fumigator knives that delivered 150 gal/acre at 3 mph directly to the root zone 8 inches off center on both sides of each hill. Thimet was applied 8 inches off center to both sides of each hill using a tractor-mounted, chain-driven Gandy applicator and incorporated into the soil with a rolling cultivator after plant emergence on 19 Jan. Vines were killed on 14 Mar with Paraquat Plus (1 qt/acre; 29.1% [AI]/acre). Two 25-ft sections of row per treatment plot were harvested on 7 Apr. Tubers were washed, graded for wireworm injury (holes less than threequarters' inch in depth), damage (holes greater than three-quarters' inch in depth), cracking, sunburn, disease, etc., and weighed fresh. Data for injury damage, sunburn, and cracking are presented as a percentage of total fresh weight/section of row. Before planting, 190 soil samples and 72 baited traps were removed to characterize the wireworm species complex in the field.

Use of a computer simulation model to evaluate a plant disease biocontrol agent

A computer simulation model (INHIBSIM) was developed to predict population dynamics of a bacterial biological control agent ( Pseudomonas fluorescens) on the foliage of red pine ( Pinus resinosa) seedlings, and subsequent effects on conidial germination of a fungal plant pathogen ( Gremmeniella abietina). The model was tested under controlled-environment and field conditions. Recoverable populations of bacteria were extremely sensitive to leaf wetness but relatively insensitive to temperature. Germination of conidia was insensitive to small variations in bacterial concentration. Successful biological control of foliar plant pathogens using bacterial antagonists will require identification of agents that do not require dense concentrations to be effective and development of techniques (formulation, environmental manipulation) to enhance their survival. Predictive population models can identify and quantify these requirements, and reduce the costs of evaluating and developing potential biocontrol agents.