G. M. Dal Bello

Grain loss caused by Tribolium castaneum, Sitophilus oryzae and Acanthoscelides obtectus in stored durum wheat and beans treated with Beauveria bassiana

The effect of the fungal entomopathogen Beauveria bassiana (Balsamo) Vuillemin (Hyphomycete) on the losses caused to durum wheat and beans by storage insects was investigated. Grains were infested with Tribolium castaneum (Herbst), Sitophilus oryzae (L.) and Acanthoscelides obtectus (Say). Beauveria bassiana was produced in inoculated autoclaved rice. The spore formulation (ground rice and B. bassiana) was applied to grain (wheat or bean) and shaken to evenly cover the grain. Adults of T. castaneum or S. oryzae were added to wheat and adults of A. obtectus to bean. Five replicates were set up for each treatment and controls (milled rice but no conidia). The insecticidal effect of B. bassiana was tested by measuring the fresh weight and weight loss of grains after four months of storage. Wheat grains infested with S. oryzae without the conidia was significantly more damaged by weevils than grains treated with B. bassiana. The mean fresh weight of grains with the conidia was significantly greater (18.4%) than the corresponding mean without the fungus when S. oryzae were present. Percentage weight loss decreased by 81.5% and was significantly smaller than the loss from the untreated grain. Significant differences were not found in the fresh weight of seed infested with T. castaneum or A. obtectus in treated or untreated grain nor in the percentage weight loss of grains infested with these insects, with and without B. bassiana.

Laboratory evaluation of chemical-biological control of the rice weevil (Sitophilus oryzae L.) in stored grains.

The virulence of ten different fungal isolates of: Beauveria bassiana, Metarhizium anisopliae, Verticillium lecanii and Paecilomyces farinosus to the rice weevil Sitophilus oryzae was tested. A fungal mix of the most efficient isolates, B. bassiana ARSEF 5500+M. anisopliae ARSEF 2974, which caused the highest mortality, was assayed in combination with fenitrothion at a concentration lower (3 ppm) than the normal 6 ppm. Fungal inoculation of insects was done by spraying conidial suspensions of each fungus on wheat. Insecticide formulations were added by spraying wheat. Treated and untreated insects were incubated on durum wheat. Insects were kept in a climatized chamber for 30 days. Observations were performed at 7, 14 and 30 days to record insect mortality. Highly significant differences were demonstrated for B. bassiana 5500 and 5501 and for M. anisopliae 2974. The level of mortality produced by treatments was: 6 ppm insecticide=97.50%, B. bassiana ARSEF 5500+M. anisopliae ARSEF 2974+3 ppm insecticide=74.17%, B. bassiana ARSEF 5500+M.anisopliae ARSEF 2974=50% and 3 ppm insecticide=37.50%. There was a statistically significant difference (p<0.05) among treatments.

Biological control of seedling blight of wheat caused by Fusarium graminearum with beneficial rhizosphere microorganisms

Fusarium graminearum is associated with the cereal damping-off complex which reduces germination, seedling stand and yield. Fifty-two bacterial strains and six Trichoderma spp. isolated from the wheat rhizosphere were evaluated for biocontrol of seedling blight of wheat caused by F. graminearum. Their potential as biocontrol agents was tested in vitro and in the greenhouse. Isolates varied in their ability to inhibit the mycelial growth of F. graminearum in agar plate bioassays by 0–79%. This parameter was not related with biocontrol efficacy of in vivo assays. In greenhouse trials, all isolates were initially evaluated for reducing disease on wheat cultivars Klein Centauro (moderately resistant to F. graminearum) and Pro INTA Oasis (susceptible) planted in sterilized soil artificially infested with the pathogen. Among the 25 bacteria and six fungal isolates that exhibited a pronounced suppressive effect, the most efficient 10 for both cultivars were further assayed on eight cultivars (Buck Candil, Buck Catriel, Buck Chambergo, Buck Poncho, Buck Topacio, Klein Cacique, Klein Centauro and Pro INTA Oasis) potted in cultivated–inoculated soil. Three weeks after sowing, plant stand, percentage of diseased emerging seedlings, plant height and dry weight were evaluated. Among the antagonists only Stenotrophomonas maltophilia was significantly better than the control for the average of the eight cultivars for plant stand, height and dry weight. Stenotrophomonas maltophilia also caused a non-significant decrease in the percentage of diseased plants. Three strains of Bacillus cereus and one isolate of Trichoderma harzianum gave also a good control in some cultivars. The ability of these isolates to affect the infection of wheat seedlings by F. graminearum may be of potential value in field trials.

Biocontrol efficacy of Trichoderma isolates for tan spot of wheat in Argentina

Abstract Pyrenophora tritici-repentis (Died) Drechs. (anamorph= Drechslera tritici-repentis (Died) Shoem. is one of the most important and widespread necrotrophic pathogen of wheat in Argentina. Breeding, chemical treatments and appropriate cultural practices are the main ways for disease control. The possibility of biological control is added as a complementary strategy within the integrated management of the disease. Trichoderma spp. have been used as biocontrol agents to protect plants against soil-borne and foliar diseases in several crops. The objective of the present work was to evaluate the potential of Trichoderma harzianum (isolate Th15, Th11, Th2, Th81, Th7, Th13, Th8, Th5), Trichoderma aureoviride (isolate Ta1, Ta100) and Trichoderma koningii (isolate Tk11, Tk6) as biocontrol agents of D. tritici-repentis under in vitro and greenhouse conditions. Dual cultures in Petri dishes containing potato dextrose agar showed that the isolates of Trichoderma spp. tested inhibited significantly the mycelial growth of D. tritici-repentis between 50% and 74%. Microscopic examination of cultures of Trichoderma spp. and D. tritici-repentis in close proximity showed plasmolysis of conidia and hyphae of the pathogen. The results of the greenhouse tests in 2000 and 2001 indicated that seven strains of Trichoderma spp. (Th5, Th11, Th13, Tk6, Tk1, Th2 and Th81) significantly reduced the disease severity on wheat plants compared with untreated plants. In general, there was a significant decrease in Trichoderma spp. population on the wheat phylloplane with time. Additional greenhouse studies using other isolates and under a wide range of temperature conditions are needed to fully assess the potential and limitations of Trichoderma spp. as biocontrol agents of D. tritici-repentis .

Increased mortality of Acanthoscelides obtectus by alkane-grown Beauveria bassiana

The effect of alkane-growth induction of theentomopathogenic fungus Beauveria bassiana(Balsamo) Vuillemin (Deuteromycotina:Hyphomycetes), on the ability to kill the beanweevil Acanthoscelides obtectus Say(Coleoptera: Bruchidae), was tested. Adultinsects were sprayed with an 0.01% Tween 20aqueous suspension of 4 × 106 conidia/ml.The performance of fungi grown in complete agarmedium containing glucose as carbon source(FS0) was compared to that of alkane-grownfungi (FS1) with n-hexadecane as the onlycarbon source. Mortality increased (p< 0.05) from 22 ± 4.5% to 44 ±11.4% at day 7, and from 26 ± 5.5% to 60± 7.1% 14 days after treatment withFS0 or FS1 respectively. The insectepicuticular hydrocarbons were analysed bycapillary gas chromatography (CGC); majorcomponents were saturated hydrocarbons, 27 to29 carbons in length. A variety ofmethyl-branched isomers of C27 were theprevailing structures, and nC27 was the majorstraight chain component. Whole insecthydrocarbons were qualitatively identical tothose of the epicuticular surface. Oleic,linoleic and palmitic acids accounted foralmost 88% of the fungal fatty acids,irrespective of the carbon source used forgrowth; however, the unsaturated/saturatedratio diminished markedly from 4.32(FS0) to 2.47 (FS1). These resultsindicate that alkane supplementation of culturemedia might be a tool to improve the virulenceof some mycoinsecticides.

Pathogenicity of Beauveria bassiana for adults of Tribolium castaneum (Col.: Tenebrionidae) in stored grains

Conidia ofBeauveria bassiana were evaluated for activity against adults ofTribolium castaneum, under laboratory conditions closely resembling the stored-grain conditions.Three doses of the fungus were applied in the treatments: 0.1 g, 0.5 g and 1 g per 20 insects/dose/flask. Treated insects and untreated were incubated in broken wheat. There were 5 replications for each test.A total of 1600 treated and control insects used were kept in a climatic chamber (70±5% and 27±2°C) for 45 days. Observations were performed daily and the recording of infected cadavers began when the percentage of mortality exceeded 50%. The evaluation of the efficiency of each dose ofB. bassiana was made 14, 21, 28 and 45 days after inoculation.Mortality of adults that were exposed to the beetles treated with 0.5 g and 1 g of conidia ofB. bassiana / 20 adults was 87 and 85% respectively, within 21 days of exposure. Onset of mortality was further delayed at lower dosage of the fungus; 55% after 21 days. After 28 days there were no difference in mortality between the 0.5 g and 1 g dosages ofB. bassiana. The most efficient treatment was 0.5 g/20 insects, which resulted in a control higher than 50% 14 days post treatment.RésuméL’activité des conidies deB. bassiana vis-à-vis d’adultes deTribolium castaneum a été évaluée dans des conditions de laboratoire similaires à celles existant dans des grains stockés.Les traitements ont consisté en une application de trois doses de champignon: 0,1 g, 0,5 g et 1 g dans des flacons contenant 20 insectes par dose, avec du blé concassé pour les témoins comme pour les individus traités. Les insectes utilisés (au nombre de 1600) ont été placés dans une chambre climatisée (70±5% et 27±2°C) pendant 45 jours. Des observations quotidiennes ont été faites et le contrôle des individus morts infectés a commencé quand le pourcentage de mortalité a dépassé les 50%. L’évaluation de l’efficacité de chaque dose deB. bassiana a été faite 14, 21, 28 et 45 jours après l’inoculation. La mortalité des adultes traités avec 0,5 g et 1 g de conidies deB. bassiana/20 adultes a été respectivement de 87 et 85%, dans les 21 premiers jours d’exposition. Le début de la mortalité a été retardé avec des doses plus faibles du champignon: seulement 55% après 21 jours d’exposition. A partir de 28 jours, il n’y a pas de différence de mortalité observée entre les doses de 0,5 g et de 1 g deB. bassiana. Le traitement le plus efficace a été obtenu avec une dose de 0,5 g / 20 insectes et a produit une mortalité supérieure à 50% après 14 jours.

Biocontrol of Acanthoscelides obtectus and Sitophilus oryzae with diatomaceous earth and Beauveria bassiana on stored grains

Abstract Combined treatment with Beauveria bassiana, and diatomaceous earth (DE) was evaluated against the bean weevil Acanthoscelides obtectus and rice weevil Sitophilus oryzae. DE from Argentina was screened both alone or in combination with water or dry fungal formulations. DE killed 100% of A. obtectus and 68% of S. oryzae showing a significantly higher insecticidal effect than the fungal dust. For A. obtectus, median lethal time (MLT) with the DE–dry fungus was significantly lower than with a fungal aqueous-suspension. In S. oryzae, powder formulations with either of B. bassiana or DE showed a MLT significantly higher than the wet treatments.

Antagonistic effect of soil rhizosphere microorganisms on Bipolaris sorokiniana, the causal agent of wheat seedling blight

The biocontrol efficiency of Epicoccum purpurascens, Gliocladium roseum, three strains of Bacillus subtilis, and Pseudomonas fluorescens, isolated from the rhizosphere of wheat plants, was assessed in relation to seedling blight caused by Bipolaris sorokiniana. An in vitro study of the potential antagonist was performed using the dual culture technique and by ‘sowing’ wheat seeds pelleted with the saprophytes in plates with water agar + the pathogen. In vivo assays were carried out in the greenhouse and in the field with pelleted seeds sown in artificially infested soil. Both the number of living plants and the number of plants with necrosis on the leaves and the base of the stems and roots were assessed 15 days after sowing. Under greenhouse conditions, B. subtilis 3 and G. roseum reduced the level of infection of Buck Pucará and Trigomax 100 cultivars, respectively. In the field, biocontrol of the disease was not achieved.

First report of Trichothecium roseum causing postharvest fruit rot of tomato in Argentina

Postharvest fruit rot caused byTrichothecium roseum on tomato fruit is recorded for the first time in Argentina. The fungus was consistently isolated from diseased fruits and shown to be the causal agent according to Koch’s Postulates. Although the economic impact of the disease is minimal,T. roseum is known as a producer of mycotoxins. Therefore, its presence on vegetables and fruits decreases the quality of those food products and causes a risk to the consumers.

Relationship between an In Vitro and Greenhouse Assay for Biological Control of Bipolaris sorokiniana-Induced Seedling Blight of Wheat

ABSTRACT Bioassays were conducted under greenhouse conditions to test the efficacy of selected bacterial and fungal antagonists applied to wheat seeds for protection against seedling blight caused by Bipolaris sorokiniana. Initially, 120 wheat rhizobacteria from soils collected at five wheat fields in Argentina were screened by two different paired culture methods. Seven isolates of Trichoderma spp. that previously showed biocontrol activity against necrotrophic pathogens of wheat were also evaluated. The in vitro tests, based on the ability of the antagonists to inhibit mycelial growth of the pathogen, were used to select the most effective isolates for further testing in a plant bioassay conducted on wheat seedlings. Disease severity, disease incidence and dry weight were estimated in the greenhouse 3 weeks after sowing. Out of 120 bacteria, 33 showed moderate to strong inhibition to the mycelial growth of B. sorokiniana on potato dextrose agar in Petri dishes. Thereafter, dual cultures in flasks containing nutrient broth showed that four strains of Bacillus cereus and one isolate of Stenotrophomonas maltophilia exhibited the greatest inhibition (> 77%) to the fungal biomass. All Trichoderma spp. inhibited the mycelial growth of the pathogen by between 51 and 71%. Seed microbiolization failed to supress seedling blight under greenhouse conditions. Nevertheless, Trichoderma isolate Th2 as well as guazatine, significantly (p = 0.05) increased plant dry weight compared with untreated pathogen-infested seeds. Suitability of in vitro methods to evaluate potential efficacy of antagonists as biocontrol agents is discussed.