First Report of Raceme Blight Caused by Cladosporium cladosporioides on Macadamia Nuts in South Africa

Abstract

In September of 2005 and 2006, macadamia (Macadamia integrifolia Maiden & Betche) orchards in Tzaneen, Modjadji, Politsi, and Levubu in the Northern Province and Kiepersol in Mpumalanga, South Africa were surveyed and sampled to determine the causal agent of raceme blight. Symptoms appeared during early bloom and were present on racemes of all developmental stages. Early signs were necrotic tips of the peduncle that often curved to one side with necrosis spreading upward, resulting in the so-called “rat tail”. Unopened flowers were also affected. In severe cases, the entire inflorescence (flowers and peduncle) was necrotic and eventually flowers abscised. Occasionally, infection began with single flowers as small water-soaked specks on the flower, with no symptoms on the green peduncle. Diseased racemes were covered with olive gray patches of mycelia and abundant conidia. Flowers with blight symptoms were collected, surface disinfested with 70% ethanol for 2 min, and left to dry. Thirty isolations were made from the interface of the lesion and healthy tissue, plated onto 50% potato dextrose agar (PDA) (Biolab, Merck Laboratories, Wadeville, South Africa) with 19 g of agar per liter, and incubated at 25°C for 5 days. Direct isolations from diseased material were done by picking up conidia and placing them on PDA. A fungus was isolated consistently and identified morphologically as Cladosporium cladosporioides (Fresen.) de Vries based on the velvety olive-brown with almost black reverse colony color and dimensions and color of conidia and conidiophores. Conidia formed in long branched chains that readily disarticulate, mostly aseptate, elliptical to limoniform, 3 to 10.5 (3 to 7) × 2 to 5 (3 to 4) μm. Conidia were pale to olive brown and smooth to verruculose. Ramoconidia were 0-1 septate, 2.5 to 5 μm wide, up to 28 μm long, smooth or sometimes minutely verruculose. Conidiophores were pale to olive brown, macro- and micronemateus, smooth or sometimes verruculose, and of various lengths up to 320 μm long and 2 to 6 μm wide. To confirm pathogen identity, the ITS 1 and ITS 4 regions were sequenced, which had 100% homology to the 18S rRNA of C. cladosporioides (GenBank Accession No. DQ 124142.1). Pathogenicity trials were conducted in the field. Fungal isolates were grown on PDA for 6 days, spores were harvested, and a suspension was made (106 spores ml–1). Twenty macadamia inflorescences (cv. Beaumont) were dipped in the suspension for 1 s, covered with plastic bags containing wet cotton wool, and covered with paper bags. Inflorescences in different stages (petal fall, knee stage, and closed) were inoculated. Control treatments were dipped in sterile water. After 2 to 3 days, the bags were removed. Symptoms developed on all 20 inflorescences and in all cases, the bottom of the inflorescence blighted, resulting in the typical rat tail symptom. C. cladosporioides was reisolated from all surface-disinfested infected material plated on PDA. Control inflorescences developed no symptoms. Isolate PPRI 8376 was deposited with the National Collection of Fungi, Plant Protection Research Institute, Pretoria, South Africa. The disease is prevalent during wet periods and 5 to 10% of flowers were infected. The disease has increasingly been seen in orchards over the last two seasons and under favorable wet, humid conditions, severe infections have resulted in 100% flower loss. To our knowledge, this is the first report of C. cladosporioides causing raceme blight on macadamia in South Africa.