Occurrence of Phomopsis sp. causing cankers on pecan trees in Buenos Aires province, Argentina

Production of pecan (Carya illinoiensis) continues to increase in Argentina, due mainly to its nutritional qualities and sustained export demand. In April 2014, the presence of cankers on branches and twigs was observed on pecan trees in the province of Buenos Aires, where the incidence ranged from 10 to 50% of the surveyed plantations (Fig. 1). Fungal isolates with morphological characteristics similar to those of Phomopsis (Sutton, 1980) were collected from cankers on potato dextrose agar after seven days of incubation in a growth chamber at 25°C with a 12 h photoperiod. Colonies in their early stages were whitish cottony (Fig. 2). They contained numerous, globose, brown to black pycnidia of 346.1 ± 59.8 µm × 286.4 ± 55.6 µm (Fig. 3), alpha hyaline conidia, fusiform to ellipsoid in shape, bigutulate with an average size of 6.8 ± 1.0 µm × 2.0 ± 0.2 µm, and beta filiform hyaline conidia, hamate in shape with average size of 21.6 ± 3.9 µm × 1.0 ± 0.1 µm.

Nucleic acid was extracted using a Promega Wizard genomic DNA purification kit (Madison, WI, USA) and the ITS region was amplified using the universal primers ITS4 and ITS5. The resulting sequence of 950 bp (GenBank Accession No. KU359781) was compared to other sequences and was most similar to Diaporthe middletonii (KJ197286, 99%), D. beilharziae (GQ250220, 98%), D. acaciarum (KP004460, 98%), D. infecunda (KC343132, 98%) and D. sackstonii (KJ197287, 97%). However, the biological and morphological characters of the Diaporthe colonies isolated from pecan were most similar to the Phomopsis anamorphic state of D. infecunda (Gomes et al., 2013). It seems likely the isolated fungus belongs to a species of Phomopsis not previously recorded on pecan. To confirm the pathogenicity of the isolates, the toothpick inoculation method was used. The inoculum was introduced into branches and twigs of pecan plants through a small incision made with a scalpel. Plants were covered with a plastic film and incubated in a growth chamber at 25ºC for 30 days. Typical disease symptoms were observed 25 days after inoculation on branches and the inoculated pathogen was re-isolated, thus confirming pathogenicity to pecan and fulfilling Koch’s postulates. The culture has been deposited at the La Plata Spegazzini Colección de Cultivos Argentina (Accession number 1219).

In Argentina, the Phomopsis genus has been reported previously on pecan nuts (Comerio, 2007). In this case, we report Phomopsis sp., anamorphic state of Diaporthe, as the causative agent of cankers on pecan for the first time in Argentina. Further studies will be necessary to fully resolve the taxonomy of Phomopsis sp. associated with cankers on pecan.