Development of a semiselective medium for isolating Xanthomonas campestris pv. musacearum from insect vectors, infected plant material and soil

A semiselective medium was developed for isolating Xanthomonas campestris pv. musacearum ( Xcm ) from infected banana plants, soil and insect vectors. The new medium was named cellobiose-cephalexin agar (CCA) and it contained (L⁻¹): 1 g yeast extract, 1 g glucose, 1 g peptone, 1 g NH₄Cl, 1 g MgSO₄·7H₂O, 3 g K₂HPO₄, 1 g beef extract, 10 g cellobiose, 14 g agar, 40 mg cephalexin, 10 mg 5-fluorouracil and 120 mg cycloheximide. The medium was evaluated for selectivity using 21 bacterial isolates and for plating efficiency using Xcm . The bacterial isolates included a soilborne Xanthomonas species and three pathogenic Xanthomonas strains that infect cassava, cabbage and beans. Although the plating efficiency of Xcm on CCA was lower (59%) than on non-selective yeast extract peptone glucose agar (YPGA), its selectivity was significantly higher, averaging 60 and 82%, when isolating from banana fruits and soil, respectively. CCA was also superior when isolating Xcm from insect vectors, with selectivity of 48–75%, compared with 8–17% on YPGA. Xanthomonas campestris pv. phaseoli did not grow on CCA, while X. campestris pv. campestris and X. axonopodis pv. manihotis grew, but their colonies were smaller than those of Xcm . Twenty-nine out of 33 suspected Xcm strains isolated from plants, soil and insects using CCA were pathogenic when inoculated onto banana plants, indicating that CCA can be a reliable tool in isolating Xcm populations. The medium should prove useful in studies on ecology, epidemiology and management of the banana bacterial wilt pathogen that is currently ravaging bananas in East and Central Africa.     

Variation in clove (Syzygium aromaticum) germplasm in the moluccan islands

Summary One hundred clove trees from a total of 34 wild and cultivated populations, mainly in the north and central Moluccas, were examined for morphological variation. Data were evaluated using cluster analysis, principal co-ordinates analysis and canonical variates analysis. Considerable morphological variation was evident and was largely attributable to between-population sources. There was more variation among wild cloves than among cultivated populations. Several of the cultivated populations were shown to be morphologically distinct. Flower bud characters were most valuable in distinguishing between populations; leaf and flower bud cluster characters were also useful. Permanent germplasm collections are needed to conserve variation.     

Weed Control

Abstract

The white powdery scale Cribrolecanium andersoni (Newstead), which occurs on citrus in South Africa and Swaziland, is of economic importance. The scale secretes honeydew on which sooty mould develops. Parasitoid sampling entailed the collection of C. andersoni‐infested leaves at seven different localities. Female and male Euxanthellus philippiae Silvestri (Aphelinidae), the female Coccophagus pulvinariae Compere (Aphelinidae), Metaphycus spp. (Encyrtidae), an unidentified encyrtid (probably a new genus), Tetrastichus sp. (Eulophidae) and a male Coccophagus sp. were found to be parasitoids of C. andersoni. E. philippiae and the unidentified encyrtid appeared to be the most common parasitoids of C. andersoni. No parasitoids were collected from first‐instar C. andersoni. The number of parasitized C. andersoni was the lowest during October to December.