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Monoconidial isolates of the pecan scab fungus, Cladosporium caryigenum, were obtained in 1993 and 1994 from one pecan orchard each in Jeff Davis and Troup counties in Georgia, counties with no previous history of exposure to demethylation-inhibiting (DMI) fungicides. Isolates were grown on potato dextrose agar (PDA) amended with propiconazole or fenbuconazole at 0,0.0001, 0.0005, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1.0, or 5.0 µg ml-1. After 4 to 5 weeks at 25°C, the diameter of each colony was measured. Relative growth was expressed as the colony diameter for each fungicide concentration as a percentage of the diameter on unamended PDA. (ED)50 values for both fungicides in all years and locations were lognormally distributed. Mean (ED)50 values ranged from 0.19 to 0.30 µg ml-1 for fenbuconazole and 0.12 to 0.17 µg ml-1 for propiconazole. There were highly significant positive correlations between sensitivity to propiconazole and sensitivity to fenbuconazole at all locations and years, indicating significant potential for development of cross-resistance to these compounds. A discriminatory concentration of 0.2 µg ml-1 propiconazole was selected for further monitoring of DMI sensitivity. With this discriminatory concentration, a sample size of 50 isolates was estimated to be sufficient to detect a difference of 8.3% in mean relative growth between two populations.

From 1989 through to and including 1994 extensive premature defoliation of black locust ( Robinia pseudoacacia L.) was observed in a progeny test plantation in Greene County, in the Georgia Piedmont. During July 1991 and 1992, all observed trees suffered greater than 50% defoliation. Leaflets of black locust showing symptoms of leaf spotting began abscising as early as mid-June following wet springs. Onset and progress of defoliation was followed on 50 trees in two plots in 1993 and 1994. In both years, sample trees were cut at the base and allowed to regrow from sprouts. A protectant fungicide, chlorothalonil (Daconil WDG 1.5 g/l water) was applied to run-off weekly to one-half of the growing sprouts beginning 17 April through 22 August 1994 by backpack sprayer. Air temperature, relative humidity, rainfall, and leaf wetness were monitored. Abscission of unprotected leaflets in 1993, a dry, hot year, began in mid-July and averaged 36% by late August; abscission of unprotected leaflets in 1994, a wet, cool...

L'optimisation mathematique peut etre un outil precieux pour la gestion des maladies. Les programmations dynamique et lineaire font partie des techniques d'optimisation. Cependant, la programmation lineaire est d'une utilisation limitee pour decrire des systemes biologiques a cause des restrictions sur la structure des modeles, tandis que la programmation dynamique (DP) est mieux adaptee pour resoudre des problemes de gestion a plusieurs etapes. Les quatre composantes de tous les modeles DP sont les periodes de decision, les variables d'etat, une fonction de transition et une fonction critere. Une decision sur la gestion de la maladie est prise a chaque periode de decision. La decision peut etre binaire (ex : traitement ou non) ou elle peut etre constituee d'un choix entre differents moyens de lutte ou differentes doses. Les variables d'etat decrivent l'etat du systeme a tout moment et peuvent, par exemple, correspondre a des periodes clefs du cycle vital de l'agent pathogene. Le nombre de variables d'etat devrait etre aussi petit que possible, par exemple moins de 10, pour simplifier les calculs. La fonction de transition decrit le changement dans les variables d'etat d'une periode de decision a la suivante comme une fonction de l'etat du systeme avec ses decisions de gestion. Elle doit avoir la propriete markovienne. La fonction de critere decrit la quantite a etre optimisee. Etant donne que les decisions concernant la lutte aux maladies sont basees essentiellement sur des profits plutot que sur le rendement ou l'intensite de maladie, la fonction critere peut etre decrite comme la maximisation des profits ou la minimisation des couts. Le modele DP foournit un mecanisme de retroaction pour assurer des decisions optimales, peu importe l'etat actuel du systeme ou les decisions precedentes. Des exemples numeriques simples de DP deterministe et de DP stochastique sont presentes pour demontrer l'efficacite des calculs par cette technique, la propriete d'optimalite et l'applicabilite de la technique dans la prise de decision pour la gestion des maladies.

Ten 14-day-old soybean plants, cv. ‘Williams 82’ were exposed continuously to hydrogen fluoride (HF) at 2 μg F m−3 in a controlled-environment chamber. After 1 week, the plants were transplanted into a potting mix infested with Rhizoctonia solani (Rs) or Phytophthora megasperma f. sp. glycinea (Pmg), and again exposed to HF for 1 week. Immediately after transplanting, plants inoculated with Pmg and uninoculated plants were flooded for 48 h, whereas plants with Rs and uninoculated checks were not flooded. One week after inoculation, hypocotyl lesions induced by Rs and Pmg were significantly larger on plants exposed to HF. Total root length infected was increased by exposure to HF, but this effect was not significant. Dry weights of individual plant parts were not affected by HF, but shoot:root ratios were higher after exposure to fluoride in experiments with Pmg. Fluoride concentrations in the leaf blades were positively correlated with root lesion length of plants inoculated with Pmg but not in those of plants inoculated with Rs.