Factors Influencing Epidemiology and Management of Blackberry Rust in Cultivated Rubus laciniatus

Authors: JOHNSON, KEN - Oregon State University; Mahaffee, Walter - Walt

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2010
Publication Date: 5/4/2010
Citation: Johnson, K., Mahaffee, W.F. 2010. Factors influencing epidemiology and management of blackberry rust in cultivated Rubus laciniatus. Plant Disease. 94:581-588.

Interpretive Summary: The blackberry rust pathogen, Phragmidium violaceum, was first observed in Oregon in spring 2005 on both commercially cultivated 'ever thornless' blackberry (Rubus laciniatus) and feral blackberry (Rubus armeniacus). Several commercial plantings of 'ever thornless' suffered severe economic losses. This research was conducted to develop a management system for the introduced pathogen , P. violaceum, in commercial blackberries. In three seasons subsequent to 2005, all five spore stages of the pathogen were observed annually, and asexual perennation of the pathogen on old leaves or in leaf buds was not evident in the disease cycle. In field experiments,germination and infection by overwintering spores occurred mostly during April. First infections in spring were associated with wetness durations of >16 hour with mean temperatures >46°F. Trap plants placed under the bundles of collected leaves frequently developed lesions, but only one of 630 trap plants placed in a production field of 'ever thornless' became infected, indicating that the effective dispersal distance of overwintering spores is limited. In growth chambers programmed for constant temperatures of 41, 50, 59, 68, 77, or 86°F, a minimum of 6 continuous hours of leaf wetness was required for infection by urediniospores, with > 9 h required for moderate infection (>4 pustules/cm2) at 59 and 68°F. With temperature regimes simulating daily temperature variation observed in the field and averaging 41, 50, 59, 68, or 77°F, urediniospore germination and infection was highest with average temperatures in the range of 41 to 59°C; similarly, in the fluctuating temperature environment, > 9 hours of leaf wetness was required to attain moderate infection. In most years, a lime sulfur applied 2 to 3 weeks prior to bud break followed by one application of myclobutanil, a demethylation-inhibitor fungicide, applied in early May near the time if or when the initial disease symptoms appear to provided effective suppression of the summer disease epidemic.

Technical Abstract: The blackberry rust pathogen, Phragmidium violaceum, was first observed in Oregon in spring 2005 on both commercially cultivated Rubus laciniatus and feral Rubus armeniacus. Several commercial plantings suffered severe economic losses. In three seasons subsequent to 2005, all five spore stages of the pathogen were observed annually, and asexual perennation of the pathogen on old leaves or in leaf buds was not evident in the disease cycle. In field experiments, teliospore germination and infection by basidiospores occurred mostly during April. On potted ‘trap’ plants exposed for periods of one week under dense collections of dead leaves bearing teliospores, basidiospore infection was associated with wetness durations of >16 hour with mean temperatures >8°C. Trap plants placed under the bundles of collected leaves frequently developed spermagonia, whereas only one of 630 trap plants placed in a production field of R. laciniatus became infected, an indication that the effective dispersal distance of basidiospores may be limited. In growth chambers programmed for constant temperatures of 5, 10, 15, 20, 25, and 30°C, a minimum of 6 continuous hours of leaf wetness was required for infection by urediniospores, with > 9 h required for moderate infection (>4 pustules/cm2) at 15 and 20°C. With diurnal temperature regimes averaging 5, 10, 15, 20, or 25°C, urediniospore germination and infection was highest with average temperatures in the range of 5 to 15°C; similarly, in the diurnal environment, > 9 hours of leaf wetness was required to attain moderate infection. In the field, lime sulfur applied as a delayed dormant treatment significantly suppressed teliospore germination and basidiospore infection. Over two seasons, one application of myclobutanil, a demethylation-inhibitor fungicide, applied in early May near the time of spermagonial appearance provided effective suppression of the summer epidemic.