First Report of Canker Disease Caused by Diplodia olivarum on Carob Tree in Italy
Authors: Granata G. , Faedda R. , Sidoti A. 
Published: February 2011

The evergreen carob tree (Ceratonia siliqua L., Fabaceae), also called locust, is widespread in the Mediterranean Region. Carob pods have been traditionally consumed as animal and human food and seeds are mainly used in the pharmaceutical and cosmetic industries. In July 2009, symptoms of canker, branch dieback, and foliage reddening were observed on carob trees in several natural areas in the province of Ragusa, Italy. Disease incidence ranged from 5 to 80% across different sites and for most areas it was nearly 15%. All affected trees showed dark necrotic tissue in the bark, cambium, and sapwood of the trunk and branches. Cankers often girdled the stem or branch, causing wilting and death of the portions beyond the canker. Black, subepidermal pycnidia developed in and erupted through the dead bark. Fragments of discolored wood were collected from 36 symptomatic carob trees (12 trees for each area), transferred onto potato dextrose agar (PDA), and incubated for 5 days at 21°C in the dark. Fungal colonies were consistently obtained from these diseased tissues. They initially were pale, becoming gray-green and finally black. After 30 days of incubation at room temperature in the natural light, colonies produced pycnidia identical to those observed in nature. A total of 500 conidia on 10 isolates were examined with a compound microscope. Conidia were initially hyaline, smooth, oblong to ovoid, both ends rounded, and aseptate; at maturity they were pale brown, one-septate, and measured 24 to 28 × 10 to 13.5 μm (means ± S.D. = 24.3 ± 1.4 × 12.1 ± 1 μm, L/W = 2.0 ± 0.18). The nucleotide sequences of the β-tubulin (GenBank Accession No. HQ660080) and TE-1α (No. HQ660078) genes and ITS-rDNA region (No. HM028640) for a representative isolate (IMI 390972) from carob showed 100, 100, and 98% similarity, respectively, when compared with the sequences HQ660079, EU392279, and EU392302, respectively, of the ex-type isolate of Diplodia olivarum (strain CBS 121887). On the basis of morphological and molecular characters, the fungus was identified as D. olivarum A.J.L. Phillips, Frisullo & Lazzizera; teleomorph unknown (1). Two-year-old trees were wounded with a scalpel through the full thickness of the bark along 1-cm longitudinal direction and inoculated by applying a 5-mm-diameter plug of mycelial (isolate IMI 390972) on PDA to the wound site. Three control seedlings were similarly wounded and plugs of sterile PDA applied. Plugs were held in place by Parafilm. The inoculated seedlings were maintained at 20 to 22°C and a 12-h light/dark cycle. Sixty days after inoculation, all inoculated trees showed leaf chlorosis, sunken, necrotic bark at the inoculation sites and finally pycnidia of D. olivarum. All treated seedlings were killed within 6 months from the inoculation. No symptoms were observed in the control plants. The pathogen was consistently reisolated from all the inoculated trees, but not from the control plants. D. olivarum has been found on rotting olive drupes in Apulia (southern Italy) and was first described as a new species in 2008 (1). This fungal species could be phenotypically misidentified as the closely related species D. mutila, which differs by having larger mean dimensions of conidia. To our knowledge, this is the first report of D. olivarum inducing canker and dieback on carob tree.

Reference: (1) C. Lazzizera et al. Fungal Divers. 31:63, 2008.