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  • Author or Editor: Jeanne D. Mihail x
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Cane dieback and foliar necrosis caused by an unknown pathogen were observed in elderberry (Sambucus nigra subspecies) plantings in early Spring 2016. Studies were conducted to identify the causal organism and determine the effects of infection on vegetative growth and fruiting of selected elderberry cultivars under controlled conditions. A Heterophoma sp. was isolated from symptomatic ‘Ranch’ elderberry canes growing in a commercial planting in Missouri. In the subsequent 2017 experiments, all canes of ‘Bob Gordon’ plants inoculated with this Heterophoma isolate had reduced cane lengths, reduced numbers of leaves and leaflets, and failed to produce fruit. In a study using ‘Scotia’, none of the 32 inoculated canes had symptomatic tissue; but in a subsequent study, one of 16 inoculated canes had slight cane dieback, indicating a potential difference in cultivar susceptibility to the pathogen. In 2018, seven elderberry cultivars (‘Adams II’, ‘Bob Gordon’, ‘Marge’, ‘Ozark’, ‘Scotia’, ‘Wyldewood’, and ‘York’) were inoculated with the same Heterophoma isolate, which was subsequently reisolated from the boundary of symptomatic and asymptomatic tissue, thereby confirming pathogenicity of the fungus. The fungus was tentatively identified as H. novae-verbascicola (Aveskamp, Gruyter & Verkley) Q. Chen & L. Cai based on analysis of genomic DNA from the internal transcribed space (ITS) region. Only two of 16 ‘Scotia’ inoculated canes had cane dieback in 2018, whereas inoculated canes of all other cultivars developed symptoms. Cane and foliar growth of each inoculated cultivar was less than the associated noninoculated control at 14, 30, and 60 days after treatment (DAT). Among inoculated symptomatic plants, ‘Marge’ canes produced more vegetative growth than all other cultivars, and ‘Adams II’, ‘Ozark’, and ‘York’ canes generally produced the fewest number of leaves and leaflets at 60 DAT. Flowering was observed on canes of noninoculated control plants, but not on inoculated symptomatic canes by 60 DAT. Thus, the newly reported cane dieback disease of elderberry, caused by Heterophoma sp., adversely affected plant growth and fruiting of Sambucus nigra subspecies. However, ‘Scotia’ elderberry plants were less susceptible to the fungal infection than other cultivars, indicating that it may be possible to use ‘Scotia’ to develop new elderberry cultivars with improved resistance to infection by Heterophoma.

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Elderberry rust (Puccinia sambuci Schwein.) Arthur (=P. bolleyana) () disease is frequently found in commercial American elderberry (Sambucus nigra L. subsp. canadensis L.) plantings when an alternate host, Carex sp., is present. To evaluate potential infection periods of P. sambuci on elderberry plants, micrometeorological conditions were monitored. Rust symptoms were observed on elderberry on 5 Apr. 2016, and conditions favorable for possible infection were 9 to 18 °C, ≥3 hours of continuous leaf wetness, and ≥85% relative humidity. Studies were also conducted to ascertain whether P. sambuci with varying pustule numbers affects fruiting, berry puree quality, or vegetative growth. Fruit yield was reduced by 31% when potted ‘Bob Gordon’ elderberry averaged six rust pustules per plant compared with noninfected plants. In another experiment, field-grown ‘Wyldewood’ plants averaging 137 rust pustules/cane at harvest had 47% less fruit weight on canes than uninfected canes. Titratable acidity of fruit puree from plants was lower when plants had either 690 rust pustules/plant or 137/pustules/cane, but soluble solids and pH of puree were unaffected by P. sambuci infection. The effect of rust infection on vegetative growth of elderberry plants also varied with pustule numbers. With a low infection level (six pustules per plant), P. sambuci did not induce premature leaf loss on ‘Bob Gordon’ plants or adversely affect shoot dry weight at the end of the growing season. When P. sambuci infection on ‘Wyldewood’ plants was more severe (137 pustules/cane), greater leaf loss occurred on infected canes than on uninfected canes. At very high infection levels (690 pustules/plant), ‘Bob Gordon’ plant dry weight was reduced. Because of the potential for fruit yield loss on elderberry plants, control of P. sambuci at relatively low infection levels on this plant may be warranted. Strategies that eliminate or suppress the alternate host would likely reduce the P. sambuci inoculum and limit the potential for elderberry plant infection.

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