Dragan ŽNIDARČIČ, Stanislav TRDAN, Jože OSVALD

Agronomy Department, Biotechnical faculty, University of Ljubljana, Slovenia (E-mail: dragan.znidarcic@bf.uni-lj.si)

Introduction

Downy mildew (caused by Bremia lactuca Regel) is a destructive disease of lettuce (Lactuca sativa) in both commercial fields and glasshouse production systems in Slovenia. Primarily a foliar disease, lettuce downy mildew has a direct effect on yield and quality, since it affects the marketable portion of the crop (Raid and Datnof, 1992).

Compared to grey mould (Botrytis cinerea Pers.: Fr.) and sclerotinia rot (Sclerotinia spp.), which - during the last years - are causing the most of or at least the most obvious loss of lettuce yield (Žerjav, 2000), lettuce downy mildew (Bremia lactucae Regel) seems less important; nevertheless more intense infections with this pathogen are being determined.

Due to suitable climate in our country for lettuce growing in the open (Černe, 2000) this vegetable plant can be planted in the open early in the spring, and because the pathogenic fungus causing lettuce downy mildew (Bremia lactucae Regel) is not demanding as far as temperatures needed are concerned (Maček, 1991; Malezieux et al., 1993) non tolerant cultivars can soon become a suitable host for this pathogen. This is becoming more and more evident since new pathotypes of the fungus causing lettuce downy mildew (Bremia lactucae Regel) are spreading in Europe (Cobelli et al., 1998). So, choosing a suitable (tolerant) cultivar as well as appropriate growing techniques become very important for protecting lettuce crop against downy mildew Bremia lactucae Regel (Zinkernagel, 1990; Crute, 1992), especially if we are trying to avoid the use of pesticides or at least to use them as little as possible (Meier, 1994). This was recognized already some time ago, so the lettuce cultivars tolerant to lettuce downy mildew (Bremia lactucae Regel), are an important part of cultivar lists in numerous countries, including our neighbours (Di Giovannantonio, 1998).

Materials and methods

A 2-year field trial on the tolerance of lettuce cultivars ('Lidija', 'Marija', 'Great lakes', 'Vanity', 'Dalmatinska ledenka', 'Ljubljanska ledenka' and 'Lusiana') to lettuce downy mildew (Bremia lactucae Regel) was conducted during 2000-2001 on the Laboratory field of the Biotechnical Faculty in Ljubljana.

Seeds were sown into seedling tray with fertilized peat. Young plants in the greenhouse were watered daily and fertilized once a week with a liquid fertilizer (0.75 g N, 0.55 g P₂O₅, 1.45 g K₂O/l). After the plants have developed 5-6 green leaves they were planted in the open (on April 18 2000 and on April 26 2001).

The soil type was a heavy clay loam which had been fertilized in late autumn with 3 kg of cattle manure m^-2. A total of 40 g m^-2 of compound fertilizer (15-15-15) was broadcast-applied and incorporated one week prior to transplanting. The beds were 0.15 cm high and 1.2 m wide. Black polyethilene mulch was laid by a machine.

The trial was carried out in three blocks, each containing one parcel of each cultivar. The parcels for each cultivar were 2 m long and 1 m wide, the 32 plants were planted in 4 rows (8 plants per row). The distances between the plants being 20 cm in the row and 25 cm between the rows.

The lettuce was harvested manually on June 8, 2000 and June 20, 2001. The degree of infection was evaluated according to EPPO scale scores 1-6 (1 ... no infection, 2 ... up to 5 % of infected leaf surface, 3 ... 6-10 % of infected leaf surface, 4 ... 11-25 % of infected leaf surface, 5 ... 26-50 % of infected leaf surface, 6 … more than 50 % of infected leaf surface) (OEPP/EPPO, 1997).

The results were statistically evaluated using the analysis of variance and Newman-Keuls multiple range test considering a 95 % probability.

Result and discussion

During two climatically rather different spring season differences in infection of summer lettuce with lettuce downy mildew (Bremia lactucae Regel) was established. The spring of the year 2000 was considerably drier compared to the spring 2001 and as such less favourable for the infection with the pathogen in question. A drier and warmer spring resulted in a quicker and better growth (the plants were irrigated and did not suffer from water shortage), so the yields in 2000 were more than doubled compared to those of 2001. The strong (vital) plants were very suitable hosts for the pathogen Bremia lactucae Regel.

During the more humid and colder 2001 spring the growth of the lettuce was slower and resulted in an overall lower yield. The abundant rainfall caused the water to stand on the mulch and so the humidity around the plants increased additionally. This enabled a more pronounced infection of the outer leaves with the asexual spores (sporangia) of the fungus.

Cultivars 'Lidija' and 'Marija', which exhibited an average tolerance towards the lettuce downy mildew (Bremia lactucae Regel) gave the highest average yield. Cultivar 'Lusiana' proved to be the most tolerant to lettuce downy mildew, but it gave the lowest yield among all the cultivars included in the study. The cultivars 'Dalmatinska ledenka', 'Great lakes' and 'Vanity' were among those most susceptible for the infection with the fungus Bremia lactucae Regel, but their yields were nevertheless quite satisfactory.

Figure 1: The average yield per plant (g) and the average degree of infection of the seven cultivars of lettuce (Means followed by the same letter are no significant difference between the yield per plant)

Conclusions

The study on the economic importance of the pathogen Bremia lactucae Regel on summer lettuce (Lactuca sativa L.) in Slovenia proved that though the pathogene potential of the fungus in question did increase during the last decade (obviously new, more aggressive pathotypes of the fungus are becoming more common also in Slovenia as in other parts of Europe) its economic importance remains relatively small.

The lettuce downy mildew (Bremia lactucae Regel) is known to develop on the outer leaves of the host plants, which enables a relatively normal development of the plants in spite of a relatively early infection. It became obvious that the differences between the cultivars (and consequently selection of more tolerant cultivars) is not as important as it was considered in the beginning of this study. The most infected cultivars (e. g. 'Dalmatinska ledenka', 'Great lakes' and 'Vanity') are capable of giving satisfactory yields, so their relatively pronounced susceptibility to lettuce downy mildew is finally acceptable. On the other hand the cultivars which are more tolerant toward this pathogen (e. g. 'Lusiana') show unfavourably low yields.

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