A survey of fluorescent pseudomonads associated with diseased wheat was conducted in South Africa during 1987 and 1988. Phenotypic features of 87 local strains were compared with those of 10 reference strains. Five groups were distinguished. Group 1 (nine reference and 16 local strains) was classified as Pseudomonas syringae pv. syringae. Group 2 (four local strains) was similar to group 1 but did not produce levan on nutrient sucrose agar. Group 3 (one reference and 33 local strains) also resembled P. s. pv. syringae, but did not elicit a hypersensitive reaction on tobacco. Group 4 (20 local strains) was mostly isolated from plants with atypical symptoms (total melanism) found in a single geographical region (Villiers) within South Africa. These strains had uniform characteristics, but failed to induce melanism on inoculated test plants. Group 5 (14 local strains) was not uniform. Twenty‐eight representative local strains, selected from each of the five groups, and the 10 reference strains were used in pathogenicity and virulence tests. The four most virulent local strains were used to screen 14 wheat cultivars grown commercially in South Africa. Five of the cultivars were susceptible to these strains. Symptoms on leaves of naturally‐infected plants corresponded with those already described, but the typical ear symptom (basal glume rot) was absent.
Global demand for rooibos tea is increasing whereas yields are decreasing in the primary production area of Clanwilliam, South Africa. Commercial rooibos producers report that tea yields decline over time following the initial clearing of the natural fynbos veld. Therefore, the aim of this study was to investigate soil and plant quality in cultivated rooibos plantations of various ages (1–60 years) and adjacent, wild rooibos stands in pristine fynbos in the Clanwilliam area. Soil chemical and physical properties, plant total biomass, tea yields, foliar elemental contents, extent of root nitrogen (N) nodulation and mycorrhizal colonisation were assessed. The most prominent soil quality changes at the oldest cultivated sites compared with pristine fynbos soils were an increase in soil phosphorus (P) from 1.3–1.7 mg kg–1 to 4.0–17.0 mg kg–1, a general decline in total carbon from 0.14–0.29% to 0.09–0.10% and decline in total exchangeable basic cations from 0.64–0.78 cmolc kg–1 to 0.34–0.51 cmolc kg–1, and a concomitant increase in exchangeable aluminium (Al) from 0.10–0.30 cmolc kg–1 to 1.03–1.83 cmolc kg–1. Foliar N : P ratios notably declined at the cultivated sites (12 : 0 – 20 : 1) compared with pristine fynbos sites (27 : 1 – 33 : 1), indicating foliar P accumulation and lack of N. Soil P was strongly negatively correlated with rooibos root mycorrhizal colonisation. The decline in soil organic matter and basic cations, especially potassium (K), was most strongly correlated with the decline in rooibos shoot biomass yields at cultivated sites. These findings highlight the significant role of soil quality in declining yields of rooibos tea in the Clanwilliam area. Management practices should be implemented that increase soil organic matter and essential basic cations such as K, and soil P and exchangeable Al levels should be monitored.
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