Detection of Puccinia pelargonii-zonalis and management of geranium rust in the greenhouse
Scocco, Erika Anne
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Geranium rust, caused by the fungus Puccinia pelargonii-zonalis, can result in unmarketable plants and economic losses to commercial growers. P. pelargonii-zonalis is a microcylic, autoecious rust pathogen that infects zonal geranium (Pelargonium × hortorum) (Doidge 1926; Rytter 1993). Current management of geranium rust includes scouting, use of clean stock plants, and prophylactic applications of fungicides. If rust is discovered, either neighboring plants in a 1-m2 area are destroyed if disease pressure is low or all plants are destroyed if disease pressure is high. A real-time polymerase chain reaction (qPCR) assay using the specific primer pair GRF and GRust-R2 was developed to detect urediniospore DNA. GRF and GRust-R2 consistently amplified P. pelargonii-zonalis DNA at 1 ng and 100 pg in a conventional polymerase chain reaction (PCR) assay and at 1 pg using qPCR, and the primer pair detected 100 urediniospores ml-1 using qPCR. Urediniospore lysis by beating with glass beads or by freezing in liquid nitrogen and then grinding for subsequent PCR was affected by the volume of 0.1% Tween 20 used to suspend urediniospores. Volumes of 50-600 µl of Tween 20 resulted in >95% of urediniospores macerated but increasing the volume to 750-900 µl reduced maceration to <40%. Plants with sporulating lesions produced 1,580 urediniospores over a 24-h period. A linear relationship was observed between atmospheric concentrations of urediniospores and distance from an inoculum source. At greenhouse bench level, 20 urediniospores cm-2 were deposited 1.8 m from the source plants producing inoculum after 9 h. The presence of geranium plants reduced movement of urediniospores along the bench. DNA was amplified by qPCR from urediniospores washed from a single inoculated leaf but urediniospores obtained from mixtures of one inoculated leaf among healthy leaves were not detectable using qPCR. Integrated disease management (IDM) strategies, specifically removing senescent flowers and han- watering, impacted urediniospore movement from plant to plant. A combination of no fungicide application, removal of senescent flowers, and hand-watering resulted in more than twice the number of urediniospores (40 urediniospores cm-1) moving among plants than any other combination of IDM treatments (≤ 20 urediniospores cm-1). Therefore, timely discovery of infected geranium plants is essential to limit urediniospore spread in the greenhouse. My results suggest that removal of a 1-m2 block of plants around an inoculum source may not be sufficient to eliminate all of the potentially contaminated plants.