First report of stem rot caused by Bipolaris cactivora on Echinocereus rigidissimus subsp. rubispinus in Italy

Abstract

Echinocereus rigidissimus subsp. rubispinus, Cactaceae family, is grown in Italy as an ornamental potted plant. In June 2018, a top stem rot was observed on about 70% of 200 3-year-old plants growing on a farm in Ventimiglia, Imperia province, northern Italy. Internal stem tissues became soft and blackened, and occasionally the upper part of the affected stem had a small depression or hole. Rotted tissues produced pale brown, ellipsoid to fusiform or obclavate conidia with two to four septa. Conidia measured 22 to 66 × 7 to 13 (average 45 × 10) µm (n = 50). Several fragments (about 3 × 3 mm) were excised from the margins of internal affected tissues and cultured on potato dextrose agar. A fungus that produced dark green, velvety colonies was consistently isolated. The fungus was cultured on potato carrot agar (PCA) at 22°C in an 8-h/16-h light/dark cycle and produced conidia morphologically similar to those observed in vivo. On the basis of these morphological characteristics, the fungus was identified as Bipolaris cactivora (Petr.) Alcorn [syn. Drechslera cactivora (Petr.) M. B. Ellis] (Sivanesan 1990). The internal transcribed spacer region of the rDNA was amplified for the isolate DB18GIU49 using ITS1/ITS4 primers (White et al. 1990) and sequenced (GenBank accession no. MH725590). BLASTn analysis of the 565-bp segment showed a 99% similarity with the sequence HM598677 of B. cactivora from Hylocereus undatus (Tarnowski et al. 2010). For pathogenicity tests, a conidial suspension (6.3 × 104 CFU/ml) was obtained from a pure culture of the fungus grown on PCA. Eight 2-year-old healthy plants of E. rigidissimus subsp. rubispinus were wounded on the apex, and each lesion (about 2 × 2 mm) was inoculated with a 10-µl drop of the conidial suspension. Eight nonwounded stem apexes were inoculated with the same conidial suspension, and eight wounded stems were treated with sterilized water. Plants were covered with moistened plastic bags and maintained in a growth chamber at 22 ± 1°C in a 12-h/12-h light/dark cycle. After 5 days, rot symptoms appeared on wounded, inoculated stems. Infected tissue became dark and then collapsed. A fungus was reisolated from the blackened internal tissues and had the same morphological characteristics as B. cactivora. Only two nonwounded, inoculated stems had rot symptoms, and B. cactivora was reisolated from both. The control plants remained healthy, and attempts to reisolate the pathogen failed. The occurrence of stem rot caused by B. cactivora has been described on succulent hosts such as Cereus peruvianus, Hylocereus undatus, and Portulaca oleracea (Farr and Rossman 2018). To our knowledge, this is the first report of B. cactivora on E. rigidissimus subsp. rubispinus in Italy, as well as worldwide. In Italy, the production of E. rigidissimus subsp. rubispinus is limited, but the pathogen could spread and cause severe losses of other hosts on which B. cactivora has been reported.