The epidemiology and control of Leptosphaeria maculans cause of Crucifer Blackleg, in KwaZulu-Natal.

Abstract

The perfect stage of Leptosphaeria maculans is reported for the first time in South Africa. Viable pseudothecia and pycnidia were found on dead, weathered tissue, sometimes in close association, whereas only pycnidia were found on live tissue. Some seedlots of imported cabbage seed were found to be internally infected with L. maculans at low levels and Alternaria brassicicola at higher levels. Fungicides iprodione (dicarboximide), triforine and propiconazole (sterol-biosynthesis inhibitor) eliminated both pathogens from infected seed. In a field trial of eight cabbage and two cauliflower cultivars, incidence of stem infection by L. maculans ranged from 16-80%. Two seedlots of the cabbage cultivar Gloria Osena differed in blackleg stem susceptibility. No correlation was found between stem lesion incidence and foliar infection counts of each cultivar, or stem lesion incidence and each cultivar's average days-to-harvest. In a second trial, incidence of stem infection ranged from 50% (Rotan) to 95% (Dynasty) in cabbage, and 64.2 to 96.6% in cauliflower cultivars. All Brussels sprouts and broccoli cultivars tested were highly susceptible. The cultivars of turnip and tyfon tested were observed to be immune to blackleg, whereas the swedes, Japanese radish, chou moullier and red cabbage cultivars tested were highly susceptible. No correlation was found between stem length and incidence of stem infection. Different seedlots within several cabbage and cauliflower cultivars differed in their blackleg susceptibility. A third cultivar trial with 10 replicates of four seedlots of one cabbage cultivar confirmed that different seedlots of a single cultivar may vary significantly in their susceptibility to blackleg. Benomyl was applied to cabbage at the seedling stage only, or at the seedling stage followed by field applications every 14 d. Relative to an untreated control, multiple applications of benomyl resulted in a 33% reduction in stem infection, a ten-fold reduction in plants killed and a 50% reduction in the proportion of non-harvestable heads, relative to an untreated control. Seedling treatment resulted in a lower infection level, a lower mortality rate and a greater mean head mass than those of the untreated control. However, none of these differences were statistically significant. In a debris degradation trial, more than 90% of buried debris (cabbage stems infected by L. maculans) had decomposed after 2.5 yr, whereas 80% of surface debris had decomposed over the same period. The susceptibilities of seedbed transplants (SBT) and container-grown seedlings (CGS) were compared using different forms of L. maculans inoculum. "Dunk" inoculation of SBT into a pycnidiosporial suspension resulted in a stem infection level of 50% greater than an uninocu1ated control. Contamination of seedbeds resulted in an infection level of 46%. "Dunk" inoculation of CGS resulted in infection level of 22%. When CGS were grown in contaminated trays an infection level of 33.4% resulted. Interplot interference ill the form of inoculum dispersal over a 1 m border was low (1.8 and 2.7% for SBT and CGS, respectively) . In a further trial examining the relationship of inoculum level and blackleg, a strong interaction was found between inoculation technique and inoculum level. Inoculation of field plots with infected debris was a more efficient technique than dipping seedlings into a pycnidiospore suspension prior to transplanting. Twenty nine blackleg epidemics were surveyed over 11 yr. Seedbed transplants (SBT) had been used in 83% of cases. Two cases (7%) had involved direct drilled seedlings (DDS). However, excess seedlings had been transplanted, making DDS epidemiologically equivalent to SBT. Three cases (10%) had involved container-grown seedlings (CGS) grown on mono cropped cabbage lands. Disease occurred in two patterns: in crops grown from SBT and DDS, blackleg occurred down the lines. In all CGS cases, disease occurrence was randomly patterned. In all cases, diseased debris was found in seedbeds and production fields. Disease spread in the field was limited to the two plants on either side of the initially infected plant, 1.3 m or less, suggesting that infection had resulted from splash dispersed pycnidiospores. The disease cycle was mono- or oligo cyclic but not polycyclic. Over a period of 6 yr, cabbage fields of 26 farms were each examined once for cruciferous weeds infected with L. maculans. No viable blackleg lesions were discovered on cruciferous weeds, suggesting that weeds play no role in the local crucifer blackleg pathosystem. A theory is proposed that windows of disease susceptibility open and shut during the different phenological stages of a crucifer's life, and that the susceptibility of different plant organs vary with the phenological state of the plant. It is also postulated that blackleg is a "low sugar disease". Disease incidence was lower in well fertilized cabbage plants than minimally fertilized plants. Organoleptic tests of cabbage cultivars correlated superior flavour and texture in cabbage with a high susceptibility to blackleg. An integrated management strategy is proposed, based on seed treatment with fungicides, the use of container-grown seedlings rather than seedbed transplants, a 3 yr rotation of crucifer lands with non-cruciferous crops, implementation of either deep-ploughing or accelerated biodegradation to eliminate debris, the development of higher levels of horizontal resistance to L. maculans in cruciferous vegetables, application of field fungicides in high risk areas (benzimidazoles or triazoles, or combinations), and the minimization of stress and optimization of host nutrition.