Since the relaxation of federal sanctions on the planting of Ribes crops because of the development of white pine blister rust (WPBR) immune Ribes cultivars (3), a small industry for the production of Ribes fruit for fresh and processing markets was established in New York and surrounding New England states. The majority of Ribes acreage in the region is planted to a WPBR immune black currant R. nigrum cv. Titania. From 2008 to June 2011, symptoms resembling those caused by WPBR pathogen Cronartium ribicola were observed at a large (>12 ha) R. nigrum cv. Titania planting in Preston, CT. In 2008, infection was restricted to a single field (100% incidence), but in 2009, all fields suffered from premature defoliation by late July. In 2010 and 2011, there was considerable incidence (>85%), but premature defoliation was kept in check by chemical management. Symptoms began as chlorotic lesions (0.5 to 4.3 mm in diameter) on both sides of the leaf. These chlorotic lesions had margins delineated by leaf veins and many developed necrotic (0.3 to 0.9 mm in diameter) centers on the upper surface of leaves within 2 to 3 weeks. The undersides of lesions developed blisters containing orange uredinia (0.1 to 0.33 mm in diameter) with smooth peridia that broke with the production of yellow-orange urediniospores (30 × 15 to 25 μm). Symptoms and signs were consistent with published descriptions of C. ribicola (1) and with WPBR infections on highly susceptible R. nigrum cv. Ben Alder planted at the New York State Agricultural Experiment Station in Geneva. Additional confirmation was provided by sequencing the two internal transcribed spacer (ITS) regions and the 5.8S gene (GenBank Accession No. JN587805; 98% identity with No. DQ533975) in the nuclear ribosomal repeat using primers ITS1-F and ITS4 as described previously (2,4). Furthermore, an attempt was made to confirm pathogenicity in the greenhouse by inoculating shoots of potted nursery stock of R. nigrum cv. Titania. Shoots were inoculated by rubbing leaves with either an uninfected currant leaf or a currant leaf from Preston, CT sporulating with urediniospores. Plants were subsequently misted with dH20 and covered with plastic bags for 24 h. Plants were watered biweekly and kept in a greenhouse with 39 to 65% relative humidity at 21 to 26°C. Shoots were monitored for symptom development on a weekly basis. After 3 weeks, 2 of the 10 plants inoculated with infected leaves developed chlorotic lesions and uredinia identical to those on leaves from Preston, CT, while all others remained healthy. Although not easily reproducible in a greenhouse, the breakdown of immunity in R. nigrum cv. Titania was observed for the last 4 years in Connecticut. Given that WPBR immunity was one of the conditions for legalized planting of Ribes, the breakdown of immunity has potentially deleterious implications particularly for nearby states like Massachusetts and New York, in which the Ribes industries are expanding. Moreover, Ribes growers may need to rely on chemical management programs to manage WPBR in the future.
References: (1) G. B. Cummins. Illustrated Genera of Rust Fungi. Burgess Publishing Company, Minneapolis, MN, 1959. (2) M. Gardes and T. D. Bruns. Mol. Ecol. 2:113, 1993. (3) S. McKay. Hortic. Technol. 10:562, 2000. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, Inc., San Diego, CA, 1990.
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