Conidial Survival Research Articles

Survival of Botrytis cinerea in Soil in South‐Eastern Spain

AbstractThe survival of Botrytis cinerea in sterile and unsterile soil at different temperatures and relative air humidities was investigated in south‐eastern Spain. Conidia survived only 7 days at 40∘C but, depending on relative humidity, for 30–90 days at 22∘C. High air humidity (95%) was needed to maintain soil humidity (8%) at a level that favoured conidial survival. Conidia survived better in sterile soil than in unsterile soil, probably because of the presence in the latter of soil microorganisms antagonistic to B. cinerea. Survival of conidia in environmental conditions simulating those in a greenhouse was less than 28 days. Results showed that B. cinerea conidia cannot survive over summer in south‐eastern Spain, and other primary sources of inocula are discussed.

Survival of Conidia of Clonostachys rosea on Stored Barley Seeds and Their Biocontrol Efficacy Against Seed-borne Bipolaris sorokiniana

Survival and biocontrol activity of Clonostachys rosea (isolate IK726) conidia during storage on barley seeds were investigated. The initial density of colony forming conidia on seed was 4 2 10 3 to 9 2 10 4 colony forming units (cfu)/seed. After 5 months storage at 4°C, the density decreased by less than one order of magnitude and the biocontrol efficacy against seedling blight caused by seed-borne Bipolaris sorokiniana was maintained at a significantly high level ( > 80% disease reduction) for > 5 months. Conidial survival on seeds stored at 20°C declined more rapidly than at 4°C, and biocontrol efficacy was significantly reduced after 3-5 months. However, conidia produced on solid media over 20 days survived better than conidia produced in liquid culture and conidia from solid media produced over 12 days. In contrast, when seeds treated with conidia were packed with silica gel and stored at 20°C, the cfu density decreased by less than one order of magnitude after 5 months and the biocontrol efficacy was still high after 6 months. A dose-response curve revealed that 103 cfu/seed were needed for 80% control of seedling blight. Similar control was obtained in storage experiments when approximately 103 cfu/seed were recovered from seed, indicating that conidia which survived also retained a high ability to control disease.

Dynamics of Airborne Conidia of the Gypsy Moth (Lepidoptera: Lymantriidae) Fungal Pathogen Entomophaga maimaiga (Zygomycetes: Entomophthorales)

Airborne conidia of Entomophaga maimaiga, a fungal pathogen of gypsy moth (Lymantria dispar), were sampled during 1992 and 1993 within forest canopies hosting gypsy moth populations. Conidia were occasionally abundant in the air in a site beginning the 1992 season with >20,000 egg masses/ha, but were almost undetectable when the resident gypsy moth population had declined to <100 egg masses/ha at the beginning of the 1993 field season. From third instar to pupation, presence of conidia in the air was episodic and infection in the resident gypsy moth population increased only after the first peak of abundance in airborne conidia. Conidial flux was positively associated with leaf wetness at lags of 5–14 and 16 h. Infection among larvae caged at 0.5 m above the ground was associated with leaf wetness, suggesting that moisture is critical for conidial survival and infection. Larvae caged on the ground (therefore, exposed to both resting spores and conidia of E. maimaiga) became infected throughout the field season, while, in comparison, larvae caged at 0.5 m (exposed only to airborne conidia) were infected sporadically during 1992 and virtually never during 1993. During 1992, infections of ground-caged larvae were initiated by both resting spores and conidia.

Effects of Selected Fungicides and the Timing of Fungicide Application on Beauveria bassiana-Induced Mortality of the Colorado Potato Beetle (Coleoptera: Chrysomelidae)

Abstract Four fungicides used for controlling foliar diseases of potato ( Solanum tuberosum ) were evaluated under field and laboratory conditions for their effects on the infectivity and sporulation of Beauveria bassiana when used as a control for the Colorado potato beetle, Leptinotarsa decemlineata (CPB). We investigated the direct effects of fungicides on B. bassiana -induced CPB mortality and the effect of time between fungicide and B. bassiana application. Effects of fungicide on conidial survival in the soil and on foliage were examined in the field. Significantly more larval mortality was observed when larvae were sprayed with B. bassiana than with the water control. Fungicide had no significant effect on larval mortality in the field. In the laboratory, survival of larvae was significantly lower among larvae fed fungicide-treated foliage. B. bassiana -induced mortality in the laboratory was observed only when larvae were fed foliage treated with copper hydroxide or water. Larvae fed mancozeb- or chlorothalonil-treated foliage experienced high mortality regardless of B. bassiana treatment. While there was no significant effect of fungicide on B. bassiana sporulation on cadavers in the field, a pattern emerged that indicated higher proportions of cadavers producing conidia in plots sprayed with water or copper hydroxide than in plots sprayed with chlorothalonil or mancozeb. Survival of B. bassiana conidia in the soil and on foliage was significantly greater in plots treated with copper hydroxide or water than in plots treated with mancozeb or chlorothalonil. Fungicides such as copper hydroxide may be less deleterious to the fungus than mancozeb and chlorothalonil.

Persistence of conidia of Entomophthora muscae in relation to age, temperature, and humidity

In this paper, the effect of age, humidity, and temperature on the conidial survival of Entomophthora muscae was evaluated. E. muscae was obtained from Musca domestica in a dairy in Itatiba (Sao Paulo, Brazil) and maintained in the laboratory by continuous passage through flies. Furthermore, the ability of conidia to infect flies at three temperatures (17, 21, and 27 °C), four ages of conidia (12, 72, 96, and 154 hours) and two humidities (100 and 60% RH) was evaluated. The temperature of 21 °C was the most favorable for the infection of house flies. Humidity was a cause of variation at 27 °C when the conidia were up to 12 hours old, but had no effect at lower temperatures. Conidia held at 100% RH and aged 72 hours caused no infection at 17 °C, but were infective at 21 °C. In the present study, conidia retained viability much longer than previously observed. Finally, the effect of humidity, temperature, and conidial age is discussed.

Effects of Temperature and Sunlight on Mycosis ( Beauveria bassiana ) (Hyphomycetes: Sympodulosporae) of Grasshoppers Under Field Conditions

The influence of environmental conditions on mycosis of grasshoppers caused by Beauveria bassiana (Balsamo) Vuillemin, was investigated. Despite the deposition of considerable quantities of conidia onto grasshoppers (6.7 x 10 3 colony-forming units [CFU] per nymph), B. bassiana did not significantly reduce field populations nor did it affect specific grasshopper taxa. Conditions were warm and sunny during the trial, and slopes of conidial persistence were equally poor on both grasshoppers and grass leaves. Small numbers of conidia (<2 CFU per grasshopper) were recovered from surface-sterilized grasshoppers 5-15 d after application indicating that if infection occurred, B. bassiana did not proliferate in the hemocoel. Considerable mycosis was observed in grasshoppers placed in cages in the greenhouse, but not in grasshoppers confined in cages adjacent to the field plots. Furthermore, the prevalence of disease in the greenhouse cages decreased with the sampling date but the onset of mycosis always occurred 3-4 d after collection, suggesting that environmental conditions in the greenhouse were responsible for the increased susceptibility of grasshoppers. Higher prevalence and more rapid development of disease were observed in grasshoppers placed in shaded cages (83-89%) than in cages exposed to full sunlight (0-15%) or protected from UVB radiation (1-43%); conidial survival was equally enhanced in the shaded and UVB-protected environments. Our results indicate that the poor efficacy of B. bassiana against rangeland grasshoppers was a result of conditions of temperature and light exposure (reduced grasshopper thermoregulation), and not the result of inadequate host targeting or pathogen virulence.

DEVELOPMENT OFBEAUVERIA BASSIANAFOR CONTROL OF GRASSHOPPERS AND LOCUSTS

AbstractRecognition of the potential ofBeauveria bassiana(Balsamo) Vuillemin as a control agent of grasshoppers and locusts occurred as early as 1936, in South Africa. Field testing ofB.bassianaas an inundative control agent of grasshoppers and locusts has been facilitated by development of a solid substrate method for mass-production of the fungus and has resulted in the registration of a strain against grasshoppers in the United States. In some, but not all field trials, application has resulted in substantial reductions in grasshopper populations. Numerous environmental constraints, including temperature and ultraviolet (UV) radiation, may limit field efficacy of the fungus. Laboratory studies suggest that low humidity does not limit the ability of the fungus to initiate disease. Sunlight is the major cause of mortality of conidia on leaf surfaces. The incorporation of UVB protectants in formulations can increase conidial survival; however, these have not yet been evaluated for their effects on field efficacy ofB.bassianaagainst insects. Thermoregulation by grasshoppers has been implicated in resistance to mycosis. Results of laboratory studies indicate that grasshoppers infected withB.bassianapreferentially seek temperatures between 40 and 42 °C and these temperatures are inhibitory to disease development. In field-cage trials, a higher prevalence and more rapid development of disease were observed in grasshoppers placed in shaded cages than in grasshoppers placed in cages exposed to full sunlight. In laboratory experiments simulating grasshopper thermoregulation during daylight periods, application of bothMetarhizium flavovirideGams and Rozsypal andB.bassianasimultaneously resulted in a final prevalence of disease that was greater thanM.flavoviridealone in the hot temperature environment, and equal toB.bassianaalone in the cool temperature environment. Incorporation of sublethal levels of Dimilin with conidia ofB.bassianaincreased efficacy of the fungus against grasshoppers in laboratory and field trials. Once environmental constraints are better quantified, it may be possible to overcome them through improved formulation, strain selection, genetic or phenotypic manipulation, and inoculum targeting. Ultimately, success ofB.bassianaas a microbial control agent will depend on our ability to overcome environmental and other constraints and/or to predict its efficacy under various environmental conditions.

Influence of Ultraviolet Light Protectants on Persistence of the Entomopathogenic Fungus, Beauveria bassiana

Abstract The effect of ultraviolet light (uv) protectants on persistence of conidia of the entomopathogenic fungus Beauveria bassiana was investigated in laboratory and field environments. The survival of conidia applied in water onto glass coverslips or crested wheatgrass (Agropyron cristatum) leaves was reduced by greater than 95% after 15 min exposure to uv-B radiation in a controlled environment. Substitution of oil for water increased the survival of conidia on both substrates. However, conidial survival in oil was more pronounced on glass (74% mortality after 60 min) than on leaves (97% mortality after 60 min). The decreased protection provided by oil on leaves was attributed to spreading and/or absorption of the oil by the leaf tissues. None of 21 potential sunscreen formulations were toxic to nongerminated conidia in vitro. On wheatgrass leaves, 5 of the 12 water-compatible and two of the nine oil compatible formulations enhanced survival of conidia after 3 h exposure to uv-B radiation in a controlled environment. Four water-compatible and three oil-compatible sunscreen adjuvants were subsequently tested in a repeated held experiment. The water-compatible fluorescent brightener, Tinopal LPW (conidial survival slopes of −2.1 and −1.7 in trials one and two, respectively), and a clay emulsion (slopes of −2.5 and −2.0) significantly increased survival of conidia compared to the water control (slopes of −3.3 and −2.7), whereas Congo Red (slopes of −3.1 and −2.8) and the optical brightener, Blankophor BSU (slopes of −4.2 and −3.7), were ineffective. Conidial survival in the field was not enhanced by the three oil-compatible adjuvants tested (oxybenzone, octyl-salicylate, and ethyl-cinnamate). The use of uv-B protectants in formulations can increase conidial survival and may enhance the efficacy of B. bassiana for controlling insect pests in epigeal habitats.

Glucose oxidase overproducing mutants of Penicillium variabile (P16)

Conidia of Penicillium variabile P16 were subjected to mutagenesis and selection for glucose oxidase production on media containing o-dianisidine. Studies of the relationship between dose of UV irradiation and conidial survival and frequency of mutation showed that the best frequency of positive mutation (17%) was obtained in correspondence to a conidial survival of 52%. Out of 54 overproducing mutants tested in shaken flasks, M-80.10 showed the highest level of glucose oxidase activity (127% higher than the wild-type). M-80.10 mutant, transferred every 15 days to fresh medium and tested monthly for 8 months, appeared stable. The time course of growth and enzyme production by the mutant M-80.10 showed an increase of the glucose oxidase activity in the culture medium up to 19 U ml −1 after 96 h of fermentation.

Glucose oxidase overproducing mutants ofPenicillium variabile(P16)

Conidia of Penicillium variabile P16 were subjected to mutagenesis and selection for glucose oxidase production on media containing o-dianisidine. Studies of the relationship between dose of UV irradiation and conidial survival and frequency of mutation showed that the best frequency of positive mutation (17%) was obtained in correspondence to a conidial survival of 52%. Out of 54 overproducing mutants tested in shaken flasks, M-80.10 showed the highest level of glucose oxidase activity (127% higher than the wild-type). M-80.10 mutant, transferred every 15 days to fresh medium and tested monthly for 8 months, appeared stable. The time course of growth and enzyme production by the mutant M-80.10 showed an increase of the glucose oxidase activity in the culture medium up to 19 U ml−1 after 96 h of fermentation.

Effect of Soil Matric Potential on Sporulation and Conidial Survival of Metarhizium anisopliae (Deuteromycotina: Hyphomycetes)

The effects of soil matric potentials of 0 to -15 kPa on the sporulation and conidial survival of two Metarhizium anisopliae isolates (EF25 and M703) were studied in a sandy loam soil and a clay soil. Conidial survival of EF25 declined more rapidly in wet soils (0 and -2.0 kPa) than in moderately dry soils (-10 and -15 kPa). Significant effects of soil matric potentials on conidial survival were observed at 30 days in the clay soil and at 60 days in the sandy loam soil. Sporulation by a mycelial formulation of M703 was also significantly affected by soil matric potential. In both soils, M703 sporulated little at 0 and -2.0 kPa. It produced significantly more conidia at -10 kPa than at other matric potentials in the clay soil, but sporulation was highest at -15 kPa in the sandy loam soil.

Production of conidia and appressoria by Cordana johnstonii on banana

Cordana johnstonii started to produce conidia on banana leaf spots about 1 d after wetting and, under moist conditions, continued sporulating at a constant rate for another 5 d at least. The optimum temperature for conidial production (Topt) was 21 °C. At 22°, 0·04 conidia mm−2 of lesion h−1 were produced on dead leaves, but on living leaves production was 0·3-2·3 conidia mm−2 h−1 Conidial survival was affected by dryness, with half-lives averaging 53 h on moist leaves and only 10 h on dry leaf surfaces. On detached banana leaves, most appressoria were produced at 22–25° but some were produced at temperatures as low as 10° if leaves were wet. Statistical models were developed to describe the relationships between production of conidia and appressoria and duration of leaf wetness and incubation temperature.

Sporulation and Survival of Conidia ofCephalosporium gramineumas Influenced by Soil pH, Soil Matric Potential, and Soil Fumigation

In growth-chamber studies conducted at 5 and 15 C, sporulation of Cephalosporium gramineum on artificially colonized oat kernels on soil and survival of conidia in soil were significantly greater at soil pH 4.7 than at 5.7-7.5. Conidial survival also was greater at pH 7.5 than at 5.7-6.7, which produced a significant curvilinear relationship between soil pH and conidial survival. Sporulation of C. gramineum on oat kernels increased, whereas conidial survival decreased, as soil matric potential increased from −0.06 to −0.01 MPa

Temperature, Moisture, and Habitat Effects on Entomophthora muscae (Entomophthorales: Entomophthoraceae) Conidial Germination and Survival in the Onion Agroecosystem

Conidia of Entomophthora muscae (Cohn), collected from the onion fly, Delia antiqua (Meigen), produced secondary eonidia at vapor pressure deficits from 0.0 to 3.6, but at highly reduced rates when compared with germination in free water. Germ tubes (the invasive stage) were only produced in the free-water and saturated-atmosphere treatments. Germination rates for these treatments were temperature-dependent; the optimal germination temperature was close to 21°C. The upper and lower conidial germination limits were found to be between 26.7 and 32.2°C and 1.0 and 4.4°C, respectively. Germination and survival in the field were affected by the habitat where the conidia were deposited. Onions, carrots, and grassy border areas (the main sites of E. muscae sporulation in the onion agroecosystem) affected germination and survival differentially based on the amount of cover in each canopy and the location of the conidia within the canopy. On four experimental dates, conidia experienced nearly 100% mortality after a single day of exposure to full sun. Fungicide applications were detrimental to conidial survival in the field.

Environmental Factors Affecting Conidial Survival of Five Pear Decay Fungi

Environmental Factors Affecting Conidial Survival of Five Pear Decay Fungi

Selection of conidial longevity mutants of Neurospora crassa

Conidial survival was measured after incubation in white light at 30°C and 85–100% relative humidity. The heritable median lifespan (MLS) of the wild-type ( age 0) was 22 days. Spontaneous short-lived mutants ( age −) with MLS about 7 days occurred among both sexual and asexual progeny of wild-type at a frequency of about 10%. Radiationof conidia with near ultraviolet light increased the mutation frequency about 5–9-fold above spontaneous. The apparent spontaneous reversion frequency was about 0.1% or less. Four lines of evidence indicated that conditional defective conidiophorogenesis is a pleiotropic expression of the age − mutations. Of five age − mutants tested, all were photosensitive and remedial by vitamin E 1 exhibiting wild-type survival in darkness or after preculture with vitamin E. Selective breeding yielded variants ( age +) with about 14-day increments of MLS at each of three generations beyond the F 1. The segregation of either age − or age + in crosses to age 0 was Mendelian. A survey of mutants of 61 genes with other biochemical or morphological phenotypes indicated, as a conservative estimate, that 9 or 15% may exert a pleiotropic adverse effect on conidial longevity.

Studies on the prolonged storage of Metarhizium anisopliae conidia: Effect of growth substrate on conidial survival and virulence against mosquitoes

Metarhizium anisopliae was grown on six complex mycological media and on three types of rice at three moisture levels to determine the effect of growth substrate on conidial yield, viability, and virulence against mosquitoes immediately after spore maturation and after the storage of conidia at four different temperature-relative humidity (RH) combinations over a 1-year period. Conidial yields varied with the mycological media, but the viability and virulence of conidia against mosquitoes produced on all substrates were similar when spores were stored under the same conditions. The storage conditions were more critical to spore survival and virulence than the substrate upon which conidia were produced. The comparison of rice types for conidial production indicated that conidial yield, viability, and virulence to mosquitoes were more dependent upon the moisture level during growth and on the storage conditions that upon the rice used. The best storage conditions among those tested for the retention of both spore viability and virulence against mosquitoes were 19°C–97% RH and 4°C–0% RH.

Characterization of MMS-sensitive mutants of Neurospora crassa

Several MMS-sensitive mutants of Neurospora crassa were compared with the wild-type strain for their relative sensitivities to UV, X-ray, and histidine. They were also compared for the frequency of spontaneous mutation at the loci which confer resistance to p-fluorophenylalanine. The mutants were also examined for possible defects in meiotic behavior in homozygous crosses and for any change in the inducible DNA salvage pathways (as indicated by their ability to utilize DNA as the sole phosphate source in the growth medium). On the basis of these characterizations, the present MMS-sensitive mutants of Neurospora can be placed into three groups. The first group includes three mutants, mus-(SC3), mus-(SC13), and mus-(SC28). These are slow growers, insensitive to histidine with no apparent meiotic defects and may have reduced frequency of spontaneous mutation. In addition, their mycelial growth is sensitive to MMS but the conidial viability following MMS, UV or X-ray treatment appears normal or only slightly more sensitive than the wild-type. The second group includes only one mutant, mus-(SC15); its mycelial growth is very sensitive to MMS but the conidial survival following treatment with MMS or UV appears normal; however, the conidial survival following exposure to X-ray is significantly reduced. This mutant shows an increase (more than 10-fold) frequency of spontaneous mutation, but behaves normal like the wild-type with respect to fertility, growth rate and insensitivity to histidine. The third group includes mutants mus-(SC10), mus-(SC25), and mus-(SC29). These mutants are very sensitive to UV, X-rays and MMS and to histadine but have normal growth rates on minimal medium. Mutant mus-(SC10), but not mus-(SC25) and mus-(SC29), has an increased (11 ×) frequency of spontaneous mutation. On the basis of data presented, the MMS sensitivity of the first group of mutants cannot be ascertained to arise from a defect in the DNA repair pathways; instead, it may stem from altered cell permeability or other pleotropic effects of the mus mutations. However, it can be suggested that the second and third group of mus mutants may indeed result from a defect in the DNA repair pathways controlled by the mus genes; this conclusion is based on their cross-sensitivity to a number of DNA-damaging agents such as MMS, UV and/or X-ray, high frequencies of spontaneous mutation (mutator effects) and defects in meiotic behavior.

Effects of Temperature, Relative Humidity, And Desiccation on Germination of Nectria Galligena Conidia

Conidia of Nectria galligena Bres. are the important infective propagules in the epidemiology of European apple canker in California. However, little is known about the effects of temperature and relative humidity on conidial germination and survival. Various authors (2, 3, 5) indicate that conidia are strictly water-dispersed and that several hours of free (liquid phase) water are needed for infection (6), therefore, any periods in the orchard below 100% relative humidity may have deleterious effects on the survival of conidia and on the inoculum potential. The studies herein report the effects of temperature, relative humidity, and desiccation on germination of conidia.

Effect of humidity on epidemiological patterns of the powdery mildew (sphaerotheca fuliginea) on squash

Epidemics ofSphaerotheca fuliginea on squash in growth chambers developed somewhat better under dry than under humid conditions. The reason appeared to be the differing effects of humidity on the various stages of disease development: dryness favored processes of colonization, sporulation and dispersal, while high humidity favored infection and conidial survival. However, all these processes will occur under conditions of humidity far from their respective optima, thus enabling the development ofS. fuliginea under widely dissimilar conditions. Information is presented on the effects of temperature on colonization and sporulation.