First Report of Colletotrichum karstii Causing Anthracnose Spot on Pitaya (Hylocereus undatus) in Brazil

Pitaya, Hylocereus undatus (Haw.) Britt. & Rose, is a species of Cactaceae that has been planted as a fruit crop in Brazil. In August 2017, infected plants of pitaya with symptoms of anthracnose were obtained from the plantations in the municipality of Pelotas, Rio Grande do Sul state, Brazil. Symptoms comprised round or irregularly shaped lesions that initially appeared as reddish-orange spots and commonly coalesced into larger, dark-brown lesions in their stems. The centers of the lesions were gray-white with purple-brown borders, surrounded by a chlorotic halo where black dots would appear later. The incidence of the disease was up to 35%, and the severity ranged from 30 to 40%. Lesions were isolated from stem samples and cultured on solid potato dextrose agar (PDA) medium that had been surface disinfected (70% ethanol for 30 s and 1% NaClO for 1 min, followed by rinsing twice in sterile water and drying on sterilized filter paper). Colonies on PDA exhibited white aerial mycelia with an orange conidial mass. The color of the colony on the reverse side was light orange. Conidia (n = 100) were 12.3 to 17.1 μm (length) × 3.9 to 6.5 μm (width), mean ± SD = 13.2 ± 1.4 × 4.2 ± 0.4 μm, length/width ratio = 3.1, hyaline, aseptate, straight, cylindrical, and obtuse at the apex. Morphological features suggested that these isolates possessed the same characteristics as previously described for Colletotrichum spp. (Damm et al. 2012). Further diagnostic information was obtained by sequencing partial internal transcribed spacers (ITS), actin (ACT), beta-tubulin (TUB2), calmodulin (CAL), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), chitin synthase 1 (CHS-1), and histone 3 (HIS3) genes of a representative isolate, using the method and primers described by Damm et al. (2012). Sequences of the studied DNA regions were submitted to GenBank (ITS, MG693784; ACT, MG916907; TUB2, MH817038; CAL, MH817039; GAPDH, MH817040; CHS-1, MH817041; HIS3, MH817042). BLAST searches showed 99 to 100% identity with sequences of Colletotrichum karstii (Yang et al. 2011) (ITS, JQ005776; GAPDH, JQ948677; CHS-1, JQ005797; HIS3, JQ005818; ACT, JQ005839; TUB2, JQ005860). The phylogram constructed using combined datasets showed that isolates clustered into six distinct clades with high posterior probabilities; the isolate from this study clustered with C. karstii with 94% posterior probabilities. Moreover, pathogenicity tests were performed by adding sterile water on PDA cultures of C. karstii, and the resulting conidial suspension, adjusted to a concentration of 1.0 × 10⁵ conidia/ml, was sprayed on the stem surface of pitaya plants (six replicates). The same number of uninoculated plants was used as a control. The experiment was performed twice. After inoculation, the plants were placed in glass culture dishes and maintained at 25°C in an incubator with constant relative humidity of 80% and a 12-h photoperiod. One week after inoculation, all inoculated plants showed stem spot symptoms, which were similar to the symptoms previously observed in the field. The uninoculated plants remained symptomless. The fungus reisolated from inoculated stems exhibited the same morphological and molecular traits as the initial isolate. C. karstii has a wide host range; it was previously reported on Bombax aquaticum, Carica papaya, Eugenia uniflora, Malus domestica, Mangifera indica, and Vaccinium spp. in Brazil (Farr and Rossman 2018). To the best of our knowledge, this is the first report on anthracnose caused by C. karstii in pitaya plants in Brazil or anywhere in the world. This finding is of great importance for Brazilian pitaya production because this pathogen can severely damage pitaya plants and become a major problem for the cultivation of this crop.