Elderberry [Sambucus nigra L. ssp. canadensis (L.) Bolli] is a high-value crop that is grown and processed into products for niche markets (Charlebois et al., 2010; Mohebalian et al., 2012). Elderberry plants are fruit-bearing, multi-stemmed shrubs with compound leaves native to eastern and central North America. Inflorescences are indeterminate compound umbels, where the outer fruit is the first to mature (Zomlefer, 1994). With the increasing production of elderberry, wild germplasm with vigorous vegetative growth and a high number of umbels with large fruit size has been selected to enhance yields. ‘Wyldewood’, a cultivar that matures during the late season and has high-quality fruit, was selected near Brush Hill, OK, in 1998 (Byers et al., 2010). ‘Bob Gordon’, a productive midseason-ripening cultivar with pendulous umbels, was selected in Osceola, MO, in 1999 (Byers and Thomas, 2011).
Because of continuous elderberry production in a monoculture system, pests have become prevalent. Elderberry rust (Puccinia sambuci Schwein.) Arthur (P. bolleyana) is a common disease found on American elderberry that causes foliar and shoot distortion (Arthur, 1962; Kellerman, 1904; Warmund, 2017). Puccinia sambuci is a heteroecious fungus that requires two hosts, sedge (Carex spp.) and American elderberry, to complete its lifecycle (Mims, 1981; Saccardo, 1891). Of the five spore stages of P. sambuci, pycniospores and aeciospores develop on elderberry, whereas urediniospores, teliospores, and basidiospores develop on Carex spp. (Mims, 1981). At least 13 species of sedge have been reported as an alternate host for P. sambuci (Afshan and Khalid, 2009; Arthur, 1962).
Pycnia are the first signs observed on elderberry leaflets and petioles during early spring and appear as small yellow pustules on adaxial and abaxial surfaces of leaflets and stems. Pycnia are flask-shaped and contain receptive hyphae and pycniospores (Littlefield and Heath, 1979; Petersen, 1974). When pycniospores contact and adhere to receptive hyphae of a compatible mating type, they undergo plasmogamy, resulting in the formation of dikaryotic mycelium (Mims, 1981).
After dikaryotization, mycelia grow intercellularly on the abaxial leaf surface of elderberry plant tissue while producing intracellular haustoria to gain nutrients and develop aecia containing aeciospores (Petersen, 1974). Puccinia sambuci aecia are often observed on elderberry in May, with large yellow–orange pustules that cause deformed leaves, stems, and petioles. Aecia produce chains of aeciospores that are wind-blown to the alternate host, a Carex species (Mims, 1981).
After germination and subsequent infection of sedge leaf tissue in the summer, uredinia form on the adaxial surface of leaflets and produce urediniospores that can re-infect sedge plants (Bolley, 1889). During the late summer, uredinia develop into telia that produce two-cell, thick-wall teliospores that can withstand low winter temperatures (Arthur, 1962). During favorable environmental conditions in March or April, each cell of the teliospore germinates and produces a basidium, the site of meiosis, resulting in four haploid basidiospores that are wind-blown to elderberry plants (Petersen, 1974). Basidiospore germination occurs on elderberry tissue and a germ tube is produced, which penetrates the host directly through the cuticle and epidermis, with subsequent development of monokaryon hyphae that form pycnia (Agrios, 2005).
Although the disease cycle of P. sambuci has been described, the epidemiology of elderberry rust and the consequences of infection on host productivity have not been investigated. Therefore, studies were conducted to: 1) determine the effect of P. sambuci infection on fruiting and vegetative growth of elderberry plants; 2) compare soluble solids, pH, and titratable acidity of berry puree from infected and uninfected plants; and 3) characterize environmental conditions associated with potential rust infection of elderberry.
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