With the development of flue-cured and Burley tobacco culture in France, black root rot, caused by Chalaraelegans (Thielaviopsisbasicola), is becoming a problem. Since 1981 the Bergerac Tobacco Institute has studied the efficacy of new endotherapeutic fungicides of the triazole family. Among these, triadimenol was found to be the most effective. The treatment method consists of spraying tobacco plants with a triadimenolsolution 48 to 72 hours before transplanting. This method appears to be an economic and efficient means to control tobacco black root rot in the field. Treatment of young tobacco plants with triadimenol strongly stimulates rhizogenesis and this favours strengthening of the tobacco plants in the field.
The variability of P. tabacina A. populations was studied by using the isoenzymes of peroxidase (PO), malate dehydrogenase (MDH) and superoxide dismutase (SOD) as molecular markers. P. tabacina conidia/conidiophores from N. tabacum crops cultivated in distant areas of Europe (France and Bulgaria) were investigated during a long period of time (1978-1992). It was found that no variations of P. tabacina as a function of space and time occurred. The data point to the genetical stability of the presently spread strain of the pathogen and the lack of processes of new strain formation. However, a significant variability of P. tabacina as a function of host plant was established. Suspension of conidia/conidiophores of P. tabacina originating from N. tabacum (P 48 cv.) was used to inoculate the wild species N. repanda. First conidia/conidiophores of P. tabacina produced on the new host plant (designated as Pt/Nr) as well as those from the “classical” host, N. tabacum (designated as Pt/Nt) were analysed. Thus, two types of P. tabacina (according to their origin) were compared. It was shown that one MDH and most PO isoenzymes present in Pt/Nt were not observed in Pt/Nr; these isoenzymes repressed following the transfer were common for Pt/Nt and its N. tabacum host. Moreover, in conidia/conidiophores of Pt/Nr new isoenzymes appeared, namely one major PO, one MDH and a block of three SOD isoenzyme components. These isoenzymes were common for Pt/Nr and the new host, N. repanda. It is noteworthy that all fungus specific isoenzymes present in Pt/Nt (one PO, five MDH and seven SOD components) were conserved in Pt/Nr. The results reveal molecular mechanisms underlying relationships between host plants and obligatory fungal pathogens, such as P. tabacina. They could be used in blue mold epidemiology research.
R Rohr, A Eberhard, R Delon, JP Descombes and JM Demor
Tobacco leaf texture, appreciated by the difference of surface roughness of cured leaves, is studies with light microscopy and scanning electron microscopy (SEM). The leaf texture is obviously determined by the presence or absence of conical cellular protuberances on the adaxial side of the leaf. Considering the anatomic point of view, the leaf thickness, always more important when the leaf texture is open, is the only objective criterion which could be associated to the texture. The ultra-structural study with SEM and transmission electron microscopy (TEM) demonstrates that the expansion capacity of tobacco doesn't rely on cytological factors such as cellular reserves or debris. The expansion capacity could be inversely proportional with the relative importance of the mesophyll comparing to palisade parenchyma. On the studied material, no direct relation between the leaf texture and the expansion capacity has been noticed.
R Delon, B Cailleteau, JL Verrier, MN Tanne and M Sylvestre
A compound for activating systemic resistance (CGA 245 704), in the chemical class of benzothiadiazoles, was studied since 1993 for the control of tobacco blue mold (Peronosporatabacina A.) in seedbeds and in the field. One foliar application of CGA 245 704 at 1.6 g active ingredient/hl every 14 days protected tobacco plants against blue mold but protection was not total. Mixed with mefenoxam (CGA 329 351) at 16 g active ingredient/hl the protection is equivalent to standard Acylon¯ TC (25 % metalaxyl, 50 % maneb) applied as foliar spray at 0.160 kg/hl (40 g active ingredient metalaxyl per hl). This allows a reduction in the quantity of fungicides dispersed in the environment and the pesticide residues on the tobacco leaves. At the rate applied, no phytotoxic effects were observed in seedbeds or in the field.
Factors of stress induce important alterations in gene expression of plants. In tobacco, new molecular species (absent in healthy plants) appear in stress conditions; they may result from both gene derepression and posttran-scriptional events. These molecules can be regarded as markers of stress. Two groups of markers are observed: non-specific, which are induced by various stresses of different origin (pathogens or abiotic factors), and specific, appearing as a response to a definite stress. We established that non-specific markers in tobacco leaves are lipid peroxides, pathogenesis-related (PR)-proteins, one acidic peroxidase isoenzyme (relative electrophoretic mobility (REM) 0.34), phenylamides, and chlorogenoquinones. In tobacco seedlings non-specific stress markers are three acidic peroxidase isoenzymes (REM 0.51, 0.60, 0.72), as well as PR-proteins. Peroxidase activity is also increased as a response to all stress factors studied in both tobacco leaves and seedlings. A specific marker of endoparasitic fungal pathogenesis (ex: P. tabacina) in tobacco leaves and seedlings is the dramatic increase of ss-glucosidase activity. This response was not induced by other pathogens (bacteria, viruses) and abiotic factors. Hence, the non-specific markers are informative for stress situation independently of the nature of stress factors, and can be suggested as components of a general defence system in tobacco. Specific markers may be used for diagnostic purposes, and as a basis for engineering of stress resistant genotypes.
A Edreva, D Blancard, R Delon, P Bonnet and P Ricci
β-Cryptogein, a proteinaceous elicitor from the phytopathogenic fungus Phytophthoracryptogea, is known to induce leaf necrosis in tobacco and non-specific resistance (expressed in the perinecrotic leaf area) against a wide range of tobacco pathogens. To reveal mechanisms underlying the acquired resistance, biochemical changes in leaves of β-cryptogein-elicited tobacco were followed three, five and ten days after elicitation. The activities of peroxidase, β-1,3-glucanase and β-glucosidase, as well as the patterns of acidic pathogenesis-related (PR)-proteins were determined. The protected part (perinecrotic area) and the non-protected part (distant extra-perinecrotic area) of leaves of β-cryptogein-stem treated tobacco (cv. Xanthin.c.) were analyzed. Leaves of water-stem treated tobacco served as controls. It was shown that in the protected leaf part β-cryptogein caused significant metabolic shifts early after elicitation, persisting during the whole period studied. An important increase of peroxidase and β-1,3-glucanase activity was recorded. PR-protein components appeared that were absent in the controls. There were negligible changes in β-glucosidase activity. In the non-protected leaf part late and non-significant changes occurred. Taking into account the antimicrobial, regulatory and structure-modifying properties of the biochemical components studied, it may be admitted that β-cryptogein elicited the development of a hostile environment, i.e. a potential for plant resistance against subsequent pathogen invasion.