Peronospora hyoscyami f.sp. tabacina (blue mould of tobacco)

Blue mould is difficult and expensive to control, especially when environmental conditions favour disease development. Though resistant cultivars are available for some types of tobacco, disease control relies mainly on fungicides.

Nicotiana debneyi is the major source of resistance to Peronospora in Australia and Europe. A blue mould resistant version of USA cultivar Hicks (designated Resistant Hicks), with resistance from N. debneyi, was widely used in Europe from 1962 as a germplasm source. From 1965 USDA-bred Bel 61 lines, also with resistance from N. debneyi, were increasingly used as resistance sources in some countries. Resistant Hicks is adapted for growing seasons with few cloudy days, and its high resistance in Australia has been stable for over 40 years. The high resistance to Peronospora in cultivars derived from Bel 61 lines has been stable for more than 20 years in Europe. N. debneyi has resistance to P. hyoscyami on eight chromosomes; there is evidence that both American and Australian resistant cultivars derive some resistance from N. tabacum. Both the cultivar Resistant Hicks and Bel 61 lines possess only part of the full resistance available from N. debneyi; further progress may be possible by hybridizing these lines and incorporating resistance from other resistant Australian species (Lea, 1999). Resistance to blue mould has been introgressed in tobacco cultivars from Nicotiana debneyi. Only three of several resistance genes distributed among seven to eight chromosomes have been transferred, in order to avoid unfavourable 'debneyi' traits (Clayton, 1968). The resistance from N. debneyi could be reinforced by a factor present in tobacco (Clayton, 1968). According to other workers, resistance in tobacco includes a dominant major gene and additive minor genes (Schiltz et al., 1977; Palakarcheva et al., 1980): the major gene confers resistance to normal strains of P. hyoscami f.sp. tabacina and the minor genes act as supplementary factors that confer resistance to virulent strains only in the presence of the major gene (Palacharkeva, 1981; Vinogradov et al., 1984). Known tobacco lines with debneyi resistance to the PT1 and PT2 pathotypes of blue mould are 'Bel 61-10' and 'Pobeda'. A single dominant factor was introgressed in 'Chemical mutant' (Egerer, 1977; Palakarcheva and Delon, 1986). Other sources of resistance have been identified in N. goodspeedii (Gillham et al., 1977), N. megalosiphon, N. rosulata (Konotop et al., 1979), N. maritima (Dorossiev et al., 1978), N. velutina, N. excelsior (Gillham et al., 1977), N. exigua (Manolov, 1980), N. otophora (Gajos, 1979). Sources from exigua (Manolov, 1980) and otophora (Gajos, 1979) have been used in tobacco breeding.Resistance in tobacco increases with plant age, but is not influenced by leaf age (Reuveni et al., 1986). In genotypes resistant to blue mould, increases of lignin and phenolic compounds were elicited after P. tabacina infection (Georgieva et al., 1997) suggesting that the deposition of both compounds may strengthen cell walls and the formation of mechanical barriers against fungus attack.

Control of the disease is helped by restraining nitrogen fertilization and irrigation. Overhead irrigation favours disease development by lowering the temperature at the bottom of the canopy (Kucharek, 1987). Drip irrigation reduced the level of blue mould by 93% in comparison with overhead irrigation (Kucharek et al., 1996).

Fungicidal control of blue mould is absolutely necessary in temperate and subtropical tobacco areas, particularly in Europe, Australia and recently also in North America (Lucas, 1980). Dithiocarbamates or systemic products with a residual activity of at least 10-15 days are used. Systemic products belong to the phenylamide family (Gisi, 1992). The systemic metalaxyl is effective for control of both seedbed and field infections (Fortnum et al., 1982), but is not used for field applications, because of the risk of insurgence of resistant strains. The products are usually applied as foliar sprays, but in the seedbed the product can be soil- or water-applied (float-system).In the seedbed, the first application occurs at the fourth-leaf stage, followed by further applications, if necessary weekly, until transplanting. In the field, the first application occurs at the first rainfall or during periods of dewy nights. Further applications follow at 10-15-day intervals, until the onset of the hot weather stops fungal growth. Chemical sprays are not effective against systemic infection. Occurrences of metalaxyl resistance were observed in Cuba and other Caribbean countries in 1981(Kucharek et al., 1996). Resistance to metalaxyl has also been found in Texas (USA) and Mexico (Wiglesworth et al., 1988) and in other countries (Bruck et al., 1982; Johnson and McKeen, 1989). It appeared in Florida, USA, in 1991, and was widespread by 1995 during a major epidemic (Kucharek et al., 1996). Sporadic findings of apparent P. hyoscami f.sp. tabacina resistance to metalaxyl probably signify that under meteorological conditions that favour fungus growth, epidemics occur in spite of chemical application (Csinos and Arnett, 1980). Reliable forecasting systems for blue mould epidemics are not yet available, but guidelines for integrated control measures are being developed. Formulae based on temperature and relative humidity do not always prove satisfactory, because many outbreaks occur in climatic conditions considered adverse for pathogen development and disease expression.Iprovalicarb (an amino acid-amide carabamate) inhibits the growth of germ tubes of zoospores and sporangia, mycelial growth and sporulation of Peronosporaceae (Stenzel et al., 1998). A new approach to control blue mould is based on use of inducers of systemic acquired resistance (SAR). This particular form of plant resistance can be activated by biotic and abiotic (CGA 245704) agents and results in a systemic protection of the plant against diseases caused by fungi and bacteria (Ruess et al., 1996). Benzothiadiazole (CGA 245704) activating SAR has been investigated for the control of Pt in tobacco seedbeds. One foliar application of CGA 245704 every 14 days partially protected tobacco plants against disease. Mixed with mefenoxam (CGA 329 351) protection was equivalent to standard Acylon R TC (25% metalaxyl + 50% maneb) applied as a foliar spray. This allowed a reduction in the quantity of fungicides released to the environment, and in pesticide residues on tobacco leaves. At the applied rates, no phytotoxic effects were observed in seedbeds or in the field (Delon et al., 1998). CGA 245704 consistently reduced systemic infection of stems in field experiments in southern Italy (Piccirillo, 1999; unpublished data).

Many attempts have been made to control blue mould by induction of resistance through transgenic tobacco expressing the PR-1 protein gene (Alexander et al., 1993a, b). In some cases, systemic acquired resistance, which has been induced by a previous infection with a pathogen or by application of exogenous substances such as aspirin or 2,6-dichloro-isonicotinic acid, in turn induces resistance (Ryals et al., 1993).Constitutive expression of a beta-1,3-glucanase cDNA coding for the PR-N isoform in transgenic tobacco plants increased resistance to the glucan-containing Pt. Such plants had higher levels of beta-1,3-glucanase activity than the controls. Seven days after challenge inoculations with Pt all plants had high levels of beta-1,3-glucanase activity and the same PR-protein pattern. It has been suggested that the beta-1,3-glucanase isoform N is important for resistance of tobacco plants against P. tabacina (Lusso and Kuc, 1996). Acetone-soluble constituents in the leaf blade that belong to the diterpenoids alpha- and beta-4,8,13-duvatriene-1,3-diols (DVT) inhibit sporangial germination of P. hyoscami f.sp. tabacina (Reuveni et al., 1987). DVT level increases significantly with age in tobacco, but is not the sole factor contributing to resistance to blue mould (Rao et al., 1987).Attempts to induce resistance to blue mould have been made by inoculating the fungus in the stem. Systemic stem infection with conidia interferes with spot leaf infections (Ye et al., 1989; Pan et al., 1992). The technique has been developed with success on cigar-type tobacco (Tuzun et al., 1986). On other tobacco types, however, systemic infection is too severe for this technique to be of value (Diana and Piccirillo 1993; Piccirillo et al., 1995).