Possibilités et limites de la monoculture en relation avec l'application de diverses mesures culturales1

La monoculture du blé d'automne entraîn des baisses de rendement qui ne semblent pas dues uniquement à des causes sanitaires. Ces diminutions de rendement peuvent être atténuées par l'application de différentes mesures culturales. Un renforcement de la fumure azotée et, dans les parcelles exposées au piétin-verse, un traitement au bénomyl au début de la montaison, constituent les mesures les plus efficaces. Les effets négatifs de la monoculture ont été plus marqués aprés labour qu'en semis direct.     

Enzyme activity in cell cultures infected with herpesvirus suis]

The activity of succinate dehydrogenase (SDH) and lactage dehydrogenase (LDH) was studied in chick-embryo fibroblast cultures after inoculation of the virulent strain "A2" and the avirulent strain "MK" of herpesvirus suum. Strain "A2" reduced SDH activity, and so did strain MK, but here the decrease of enzyme activity was slower, and it did not become evident until the 24th hour. LDH activity fluctuated after "A2" infection but was generally increased, while there was no change in LDH activity, compared with uninfected control cells, after "MK" infection. When interaction of cell and virus took place in the presence of 5-iodo-2-desoxyuridine (IUDR), strain "A2" produced little change in the enzymes, but "MK" infection was accompanied by a definite fall in SDH and a slight increase in LDH. The presence of IUDR inhibited the proliferation of the virulent strain but had no apparent effect on proliferation of the attenuated strain "MK". Investigation of the enzyme activity of cells infected with Aujeszky's disease virus has revealed new biological properties of the virus, which might serve to distinguish between different strains of the virus.     

Role of acquisition and inoculation time in the expression of clonal variation for BYDV-PAV transmission in the aphid species Rhopalosiphum padi

Vector efficiency of 20 Rhopalosiphum padi clones, originating from Europe, North America and North Africa and exhibiting different types of life cycle, was evaluated by transmitting a French BYDV-PAV isolate to barley plants under five different acquisition/inoculation sequences (AAP/IAP). Differences between clones in transmission efficiency were found only when a short AAP was followed by a long IAP (6 h/120 h) and, to some extent, when a long AAP (48 h) was followed by a short IAP (6 h), but no differences were found when the conditions for virus transmission were optimal, i.e. long AAP followed by long IAP (48 h/120 h). There were no differences in transmission rates by clones of different geographical origins and with different life cycles. As a consequence, clonal variation is probably of little importance in the vector aspects of the epidemiology of PAV serotypes transmitted by R. padi , but the availability of a range of clones exhibiting transmission differences under limiting AAP or IAP conditions could be of interest for studies of virus–vector relationships.     

Clostridium piliforme infection (Tyzzer disease) in horses: retrospective study of 25 cases and literature review

Tyzzer disease (TD) is caused by Clostridium piliforme, a gram-negative and obligate intracellular bacterium. The disease occurs in multiple species. A triad of lesions, namely colitis, hepatitis, and myocarditis, is described in cases of TD in some species, such as rats and mice. We carried out a retrospective analysis of 25 equine cases with a diagnosis of TD; 24 of 25 cases occurred in foals <45 d old; the remaining foal was 90 d old. There were 12 males and 12 females; no sex information was available for one foal. The affected breeds were Quarter Horse, Thoroughbred, Arabian, Paint, and Hanoverian. Most of the cases (19 of 25) occurred in the spring. There were 9 cases of sudden death; the remaining animals had diarrhea, fever, distended abdomen, depression, weakness, non-responsiveness, and/or recumbency. Gross findings included icterus, hepatomegaly with acinar pattern, serosal hemorrhages, pulmonary edema, and/or fluid content in small and large intestine. Microscopically, all foals had severe, multifocal, necrotizing hepatitis. Necrotizing lymphohistiocytic colitis was observed in 10 of 25 foals, and multifocal necrotizing myocarditis was found in 8 of 25. Gram-negative, Steiner-positive, intracytoplasmic filamentous bacteria were observed in hepatocytes, enterocytes, and myocardiocytes, respectively. PCR detected C. piliforme DNA in the liver (24 of 24), colon (20 of 24), and heart (5 of 25). Our results indicate that necrotic hepatitis is the hallmark of TD in horses; the so-called triad of lesions is not a consistent characteristic of the disease in this species.