November 1999 Biological control of blackberry with LC0143 blackberry leaf rust fungus ISSN 1329-833X Keith Turnbull Research Institute, Frankston Common and scientific names Victoria. Most European blackberry species in Australia appear to exist as single clones. Blackberry leaf rust fungus; blackberry rust Blackberry is a Regionally Controlled Weed in all Phragmidium violaceum (Schultz) Winter Catchment and Land Protection Regions in Victoria except Mallee. Land owners in areas where blackberry is Regionally Controlled must take all reasonable steps to control it and prevent its spread on their land and the roadsides which adjoin their land. Blackberry growth pattern Blackberry has a two year growth pattern. First year canes emerge from the root crowns in spring and grow quickly to the top of the existing canopy, eventually arching towards the ground (Figure 3). Having reached the ground, these canes produce buds and roots at their tips (daughter plants) in autumn, which lie dormant over winter. In the following Figure 1. Blackberry leaves with golden summer spores of spring, the daughter plants become crowns and sprout new blackberry leaf rust. canes. Every spring, flowering canes grow in the axils of the old leaves of those canes which have overwintered (second year canes), producing flowers, fruit and seed. In winter, the second year canes die back to the crown, leaving independent daughter plants. A blackberry bramble consists of live first and second year canes plus dead material from previous years. Figure 2. Black over-wintering spores of blackberry leaf rust. Background European blackberry is a spiny bramble covering millions of hectares of bush and farm land in temperate regions of Australia with an annual rainfall of 750 mm or more. The botanical name of European blackberry, Rubus fruticosus L. aggregate, indicates a number of closely related species which for convenience are dealt with Figure 3. Blackberry growth pattern. (aggregated) under one name. In reality, there are at least seven species and a number of hybrids occurring in © State of Victoria, Department of Natural Resources and Environment 2002 Page 1 Biological control of blackberry with blackberry leaf rust fungus LC0143 Origin of blackberry leaf rust The blackberry leaf rust fungus is a very specific natural SPRING enemy of blackberry. It is commonly found where Sexual Reproduction blackberry originates, throughout Europe, North Africa Basidiospores Pycniospores Aecidiospores and the Middle East. Studies were undertaken in Europe in the early 1980s to assess its safety for use as a biological control agent in Australia. This research demonstrated that winter cold and humidity Phragmidium violaceum is specific to the European SUMMER blackberry and does not damage native species of Rubus or golden summer spores varieties of commercial raspberry and brambleberry such WINTER Urediniospores as loganberry and youngberry, but could damage some black overwintering spores (Many generations in spring, varieties of thornless blackberry which are hybrids of the Teliospores summer and autumn) (Summer, autumn and winter) European blackberry. Permission was sought to introduce a number of more highly damaging strains of the rust to Australia, but in 1984 the project came to an abrupt halt when the rust was discovered as an illegal introduction in southern Victoria. Figure 4. Life cycle of blackberry leaf rust. The “illegal” rust spread quickly throughout south-eastern Australia. Subsequent studies have shown that it is far less Other fungal diseases of blackberry damaging to the predominant blackberry species in south- The blackberry leaf rust can be confused with three other eastern Australia than the strains first selected in Europe. fungal diseases of blackberry. Permission to introduce the more damaging strains was Blackberry orange rust (Kuehneola uredinis) rejected in 1986 because of objections from people who considered blackberry useful. In August 1990 the This rust has been in Australia since at least the early Australian Quarantine and Inspection Service allowed 1940s. It appears on the lower surface of the leaf as small Victorian scientists to release one of the damaging strains, powdery, orange pustules, the size of a pin-point. These do which had been kept frozen in Europe since 1983. not have a corresponding purple blotch on the upper surface of the leaf. Blackberry orange rust attacks the older Description leaves of blackberry in late summer and autumn and often Blackberry leaf rust fungus is a defoliating disease which creates larger powdery, orange pustules up to 1 cm long attacks the leaves and is also found on flower buds, unripe that can break through the bark of second year canes. fruit and the green parts of growing canes. Septoria leaf spot (Septoria rubi) The rust appears as characteristic purple-brown blotches, 2 This fungus appears as purple-brown blotches 1 to 2 mm in to 3 mm in diameter, on the upper surface of the leaf (Figs. diameter on the upper surface of the leaf. The blotches 1 and 2). Corresponding yellow or black powdery pustules sometimes have a white centre. There is never a of spores appear on the lower surface of the leaf. Heavily corresponding yellow or black powdery pustule on the infected leaves turn brown, shrivel and fall from the canes. lower surface of the leaf. Life cycle Cercospora leaf spot (Cercospora rubi) The rust undergoes a sexual cycle on new blackberry This fungus appears as brownish blotches 5 to 7 mm in leaves in spring involving three spore types (Fig. 4). The diameter on the upper surface of the leaf. The blotches golden summer spores first appear in early November, sometimes have a purple fringe but never a corresponding have several generations on young leaves and reach yellow or black, powdery pustule on the leaf underside. epidemic level by flowering time. Summer spores germinate in the presence of moisture and infect the Releases blackberry when the germ tube enters the leaf through the One selected strain of the blackberry leaf rust, F15, that stomata (breathing pores), found only on the lower surface was known to be highly damaging to the predominant of blackberry leaves. Leaf age affects the level of infection, blackberry species, Rubus discolor, was released as a with the most susceptible leaves being the youngest, fully biological control agent in all Australian States in the opened leaves at the cane tips. summers of 1991 and 1992. Monitoring of its dispersal Summer spores are microscopic and carried by air was complicated by the presence of the illegal strain. currents, spreading the infection to other leaves, canes and plants. They have a generation time of 8 to 10 days. In late Impact summer and throughout autumn, the pustules produce Blackberry leaves that are heavily infected with blackberry black, sticky, over-wintering spores. These remain attached leaf rust turn brown, shrivel, and fall from the canes. The to any remaining leaves on the brambles and, when mature, blackberry is forced to grow new leaves which in turn are are responsible for starting the next cycle of rusting on new attacked by the rust. Repeated defoliation weakens the spring leaves. © State of Victoria Department of Natural Resources and Environment 2002 Page 2 Biological control of blackberry with blackberry leaf rust fungus LC0143 plant which must then use up reserves stored in its root the extent of existing infestations, slowed down the system. invasion of new areas and made the susceptible blackberry Data from the two long-term monitoring sites in Gippsland species less competitive with desirable vegetation. shows steady reductions in blackberry biomass, decreases in the lengths of canes and dramatic reduction in the production of daughter plants due to rust infection. A reduction in total biomass of 56% for Rubus polyanthemus over 12 years and 38% for R. leucostachys over 8 years has been recorded. Production of daughter plants by R. polyanthemus has been reduced by 96%. Figure 7. Defoliated blackberry in autumn. In the long term, species of blackberry that are resistant to the rust have greater comparative fitness and are likely to proliferate. Resistant blackberry species may become more widespread and problematic. Research is in progress on the complex taxonomy of the blackberry aggregate and the Figure 5. Rust on blackberry in summer. susceptibility of the various species and hybrids to blackberry rust. New strains of rust may be introduced in the future to tackle blackberry that is rust resistant. Investigations are also in progess on the environmental factors which may be limiting the distribution and effectiveness of the rust and the potential of other management techniques to enhance rust impact. Blackberry that is not attacked by the rust in summer and autumn should be assumed to be resistant and should be controlled by other means. Figure 6. Blackberry leaves that are severely attacked by the rust curl up and drop off. The rust causes extensive leaf-loss (Fig. 7) and can defoliate blackberry in summer and autumn. Defoliation of blackberry did not occur before release of the rust, but now occurs to a varying extent in summer every year. More light is able to penetrate through blackberry thickets, enabling other plant species to germinate and grow up through them. This change has been a slow, gradual process but canopies of blackberry, formerly fully closed, are now much more open. On agricultural land, the main Figure 8. Blackberry infestation weakened by blackberry rust advantage of the rust is that it has restricted the invasion of showing native plants growing through the blackberry. new land by established blackberry infestations. Rust spores require high humidity to germinate so Integrated control blackberry rust is more effective in the higher rainfall areas Biological control cannot totally eradicate a weed but can south of the Dividing Range in south-eastern Australia. reduce the spread and density of infestations. In few cases The impact of the rust in drier areas appears mostly to have is control achieved to the level where the weed is no longer been minor and patchy.