Evaluation of a stem inoculation technique for assessing resistance to Phytophthora cinnamomi in Leucospermum cultivars

An important aspect of integrated disease management of Phytophthora root r ot, the most serious root disease of Proteaceae in South Africa, is the use of resistant rootstocks. Most commercial cultivars are propagated from ste m cuttings that have been rooted in a rooting bed. Inherent resistance prop erties should, therefore, be present in the stems. The stem inoculation tec hnique for evaluation of resistance properties of various genotypes is desi rable because it is very efficient in terms of space, time, labour and cost . Ten isolates of P. cinnamomi Rands representing the A1 and A2 mating type s and a range of aggressive abilities were selected for stem inoculations. The effects on lesion length of depth of inoculation and position of measur ing the lesion were assessed. The stem inoculation technique was standardis ed by using superficial inoculations and measuring the lesion just below th e bark after 6 days incubation at 22 degreesC. Lesion development on five L eucospermum cultivars was compared after winter and summer inoculations wer e carried out over three consecutive years. Results showed that it is impor tant to standardise the stem inoculation technique to reduce variation as a consequence of method. The performance of some cultivars was very consiste nt over all test periods, but others demonstrated variability in lesion len gth. Assessment of resistance on results of a single test is thus not recom mended.