Biocontrol of blackberry (Rubus fruticosus L. agg.) by Phragmidium violaceum (Schultz) Winter

Abstract

Blackberry (Rubus fruticosus L. agg.) is a serious weed in Australia and New Zealand. Control is both difficult and costly. Spores of the rust pathogen Phragmidium violaceum (Schultz) Winter were released in Victoria, Australia in 1984 as a biological control agent specific to this weed. P. violaceum was first reported on blackberry in Canterbury, New Zealand in February 1990. It has subsequently been reported as being widespread over both main islands and as far east as the Chatham Islands. Nothing is known about either the races of the rust that have become established in New Zealand or the host/pathogen interactions.

The life-cycle of P. violaceum on blackberry in New Zealand was similar to that described in Chile and Australia and all spore stages were produced Susceptibility of four hosts differed depending on species and age of leaf. R. echinatus was the most common species in Canterbury and was the most infected species in the field. It was only moderately infected in detached leaf studies. In contrast, R. laciniatus was the least common species and only moderately infected in the field. However, it was the most infected species in detached leaf studies. R. ulmifolius was never infected in the field but was susceptible to two isolates of urediospores, collected from Farewell Spit and Seddonville, in detached leaf studies.

Of the three ages of leaves tested, the youngest, fully expanded leaves were the most susceptible to infection (P<0.001) compared to leaves from the middle of the cane (P<0.05) and the oldest leaves (P<0.001).

Eight putative physiological races of P. violaceum were identified provisionally from sixteen samples tested. Amongst these some isolates infected none of the host species tested and others infected all host species.

Urediospores germinated at 10-25°C with maximum germination occurring within 7-10h. Germination was most rapid, with maximum achieved within 3h, at 20°C. There was some evidence for adaptation to temperature in one sample. Teliospores germinated after 45h at temperatures between 5 and 25°C after cold water treatment (5°C) for two weeks.

The significance of host specialisation and conditions suitable for host infection are discussed in relation to biocontrol of blackberry with P. violaceum.