Occurrence of scab disease of pecan caused by Cladosporium caryigenum in Argentina

Pecan (Carya illinoinensis) is a new crop in Argentina. The planted area is expanding quickly with around 4000 h in the Pampas region, characterized by around 1000 mm annual rainfall and high humidity. In 2007 and 2008 we commonly observed typical scab symptoms on seedlings and mature trees in La Plata: small, circular black leaf spots, often coalescing, and an olive green to black mould below. Nuts, twigs and petioles also had black spots (Fig. 1).

We isolated a Cladosporium-like fungus on potato dextrose agar (PDA) from surface-sterilized pecan leaves collected from the Experimental Station of UNLP, La Plata. The conidiophores were dark brown, erect and septate (50–188 x 4–5 mm) (Fig. 2). Conidia were in chains, irregular, ovoid to ellipsoid, a few with two cells, pale olive green, (4–8 x 6–22 mm). These characteristics agree with a published description of Cladosporium caryigenum (Gottwald, 1982).

We inoculated the C. caryigenum isolate on detached leaves from one year-old seedlings, obtained from a tree selected as a source of rootstock, using a modified method described by Conner (2002). Conidia were harvested from two week-old PDA cultures grown at 24 ⁰C and a suspension in sterile distilled water adjusted to 1x10⁶ conidia/ml. The suspension was lightly brushed across the leaf surface while control treatments used sterile distilled water. All leaves were incubated for eight days at 25 ⁰C and 16 h photoperiod. One cm² pieces from the leaf centre were bleached and stained with trypan blue. Only inoculated leaves contained subcuticular hyphae and initial reproductive structures of C. caryigenum (Fig. 3) typical of a susceptible response (Conner & Stevenson, 2004). Pecan scab was reported from Paraguay by Kobayashi (1984) and there are earlier unconfirmed reports from Argentina. This, however, is the first confirmed report of Cladosporium caryigenum causing pecan scab in Argentina.