First report of black rot on apple fruit caused by Diplodia seriata in Washington State

Authors: KIM, Y - Pace International, Llc - Usa; KWAK, J - Pace International, Llc - Usa; AGUILAR, C - Washington State University; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2016
Publication Date: 6/8/2016
Citation: Kim, Y.K., Kwak, J.H., Aguilar, C.G., Xiao, C. 2016. First report of black rot on apple fruit caused by Diplodia seriata in Washington State. Plant Disease. 100:1499. doi: 10.1094/PDIS-12-15-1463-PDN.

Interpretive Summary: In July 2014, a fruit rot disease was observed on Fuji apple fruit in commercial orchards in Mattawa, WA. Fruit rot symptoms appeared to originate mainly from infections at either the calyx-end (floral parts) of the fruit or wounds on the fruit skin associated with insect injuries. Diseased fruit exhibited light-to-dark brown lesions without centric rings. At the advanced stage, the entire fruit was completely rotten, sponge to firm, and light brown. A fungus was isolated from the decayed fruit and identified as Diplodia seriata De Not., which is the cause of black rot of apple, based on the morphological characters and DNA fingerprinting. On apple fruit inoculated with the fungus, the fungus is able to cause rots similar to those observed in the field. Black rot has been reported in other apple growing regions including the eastern United States, where black rot was first reported; however, it has not been reported in Washington State, where more than 60% of US apples are produced. This is the first report of the occurrence of black rot on apple caused by D. seriata in Washington State.

Technical Abstract: In July 2014, decayed ‘Fuji’ apple fruit (Malus × domestica Borkh.) were observed and sampled from commercial orchards in Mattawa (Grant County) in Washington State. Fruit rot symptoms appeared to originate mainly from infections at either the calyx-end (floral parts) of the fruit or wounds on the fruit skin associated with insect injuries. Diseased fruit exhibited light-to-dark brown lesions without centric rings. At the advanced stage, the entire fruit was completely rotten, sponge to firm, and light brown. The growers reported that this disease was also observed on other cultivars in the vicinity of Mattawa, including Aurora Golden Gala (20-25% loss) in 2013 and Pink Lady (1-2% loss) in 2014. To isolate the causal agent, small fragments of fruit flesh from decayed fruit were excised from the margin of lesions and placed on potato dextrose agar (PDA) acidified with 0.1% lactic acid. The plates were incubated at 20°C for 7 days and sub-cultured on PDA to obtain pure cultures. The cultures were initially white with dense aerial mycelium and gradually became dark gray to olive green from the center. Conidia were aseptate, cylindric, rounded at both ends and brown to dark brown at maturity, and 10 to 16 × 21 to 26 µm (n = 50) in size. The fungus was identified as Diplodia seriata De Not. based on the description of its anamorphic stage (Phillips et al. 2007; Sutton 2014). The identities of five representative isolates were further confirmed by analysis of nucleotide sequences of the internal transcribed spacer (ITS) regions amplified using the primers ITS1/ITS4. A MegaBLAST search in GenBank showed that the sequences of all five isolates had 99% homology (E-value = 0.0) to a D. seriata sequence (Accession No. AY259094). A representative isolate was tested for pathogenicity on apple fruit. Organic ‘Fuji’ apple fruit were surface-disinfected in 0.6% sodium hypochlorite solution for 5 min, rinsed twice with deionized water, and air-dried. Each fruit was wounded with a sterilized finish-nail head (3 mm in depth and 4 mm in diameter) and inoculated by placing a 4-mm-diameter mycelial plug from the leading edge of a 4-day-old PDA culture on the wound. Control fruit were treated with PDA plugs without the fungus. There were four 10-fruit replicates for each treatment, and fruit were placed in plastic crispers and stored at 4°C for 4 weeks. The experiments were conducted twice. Black rot symptoms developed on all inoculated fruit, while no decays appeared on the control fruit. Koch’s postulates were fulfilled by re-isolating the fungus from the decayed fruit. Black rot has been reported in other apple growing regions including the eastern United States, where black rot was first reported; however, it has not been reported in Washington State, where more than 60% of US apples are produced. To the best of our knowledge, this is the first report of the occurrence of black rot of apple caused by D. seriata in Washington State.