IN-VITRO RELATIONSHIP BETWEEN DITHIOCARBAMATE RESIDUE AND STEMPHYLIUM-VESICARIUM INFECTION ON PEAR FRUIT

Abate Fetel pear fruits were collected at three different stages of de velopment and treated with one of two dithiocarbamates (thiram or meti ram) at concentrations ranging from 0.4 to 3.2 g l(-1). The residue ex pressed as carbon sulphide (CS2) was then measured using spectrophotom etry and correlated with fungicide dose and with development stage. Ge nerally thiram was found to leave higher residues on the fruits than m etiram. For each product, at equal doses of active ingredient, the amo unt of residue decreased as sampling date approached harvest. This pat tern could not be attributed to a change in the surface:volume ratio s ince there was no significant fruit size growth between the second and third sampling dates. Fungitoxicity tests showed that thiram was more effective than metiram against conidial germination, Stemphylium vesi carium with respective EC(50) values of 2.3 mg kg(-1) and 14 mg kg(-1) . Thiram was also found to be more effective than metiram in preventin g Stemphylium vesicarium infection on pears. The relationship between disease severity and amount of residue (CS2) left on the fruit after t reatment was found to be linear for metiram and exponential for thiram . However the residue resulting from the fungicide dose recommended fo r field treatment (0.7-0.8 g l(-1) active ingredient), corresponding t o a CS2 residue of less than 1 mg kg(-1), did not guarantee effective pear protection under the conditions tested. The efficacy of this trea tment in the field was related to residue accumulation following repea ted treatments, or to conditions less conducive to the disease onset t han those tested in the laboratory.