Mycelium: The mycelium is composed of branched, septate, olivaceous brown to brown hyphae (1-3 µm wide). On the host, the fungus colony is hypophyllous and maculicolous. The stromata may be poorly developed on leaves, but well developed on fruit shucks (husks) and twigs (Gottwald, 1982;Partridge and Morgan-Jones, 2003;Schubert et al., 2003).
Pecan scab is relatively easily detected based on visual inspection for characteristic symptoms of the disease on the foliage, fruit and shoots of its host (Nolen, 1926;Demaree, 1924, 1928;Littrell, 1980;Goff et al., 1996). Characteristic symptoms include dark black spots of scab with a velvety appearance on the surface of the affected organ.
Related invasive species
- Fusicladium effusum
Related Farm Practice
- Hosts
- Defoliation
F. effusum is a fungal pathogen that causes pecan scab, which can result in severe economic losses on susceptible cultivars and resulting harm to the pecan industry in areas with high rainfall where pecan is grown. The disease develops on leaves, fruits and shoots and results in loss of photosynthetic area and reduced fruit size and quality. Pecan scab can also lead to reduced fruit set in the following year due to plant stress. Fungicides used to control pecan scab are costly. It is introduced to new areas through movement of infected host material. Despite quarantine restrictions, it is likely that human-mediated transfer has occurred between the native habitats in south-eastern USA and Mexico, and locations where pecan is grown as an exotic in South America, South Africa and New Zealand. F. effusum overwinters as stroma and conidia in lesions on shoots and fruit shucks, and the conidia are dispersed in wind and rain splash. The pathogen is a threat to all pecan-growing regions with a humid, wet environment.
F. effusum affects the fruit, stems, leaves, dormant buds and catkins (Nolen, 1926;Demaree, 1924, 1928;Littrell, 1980). Defoliation and nut drop can occur if infection is severe. The symptoms are similar on all infected plant parts.
On the leaves, dark brown to black spots of scab can be observed on both the abaxial and adaxial surface of the lamina shortly after bud break, and are often associated with the veins or midrib. Spots vary in size from 1 to 7 mm in diameter, which can coalesce into larger spots. When young, the spots have a velvety appearance. As the infection ages, it turns hard and forms a dark grey or silvery to brown spot that extends through the leaf. These senescent spots can crack and drop out of the leaf, resulting in a shot-holed appearance on older leaves. The leaf lesions are a source of inoculum for the young fruit (Nolen, 1926;Demaree, 1928;Schubert et al., 2003).
Small, olive-green to black spots can develop on the young fruit early in the season. If conditions are favourable, the lesions can increase in size, coalesce and cause large areas of disease, often with a velvety appearance while the lesions are young. If the lesions are particularly large the surface can become brown and cracked. If infection is very severe and the pathogen penetrates deeper it can cause the shuck (husk) to cling to the shell of the nut. The spots may be slightly raised. On fruit, black fungal stromata may form on the spot, which can produce a dark, velvety growth of conidiophores the following spring (Nolen, 1926;Demaree, 1928;Schubert et al., 2003).
On the shoots or twigs the symptoms are similar to those on the leaves or fruit. The edges of the lesions may be slightly raised, with dark fungal growth in the centre. Twig lesions may also form stroma and overwinter, producing conidia the following spring, similar to those zone the fruit shuck (husk) (Nolen, 1926;Demaree, 1928).
Symptoms on the pedicels and bracts of the catkins and on dormant buds are reported to be slight and typified as small, black spots (Demaree, 1924).
From an economic viewpoint pecan (Carya illinoinensis) is the most important host. The other susceptible species are wild hosts in the same genus as pecan (Carya) that occupy similar habitats in the USA (Schubert et al., 2003;http://nt.ars-grin.gov/fungaldatabases). There are reports of its occurrence on Carpinus spp., but the causal agent in now considered a separate species (Schubert et al., 2003). There is an unconfirmed report of F. effusum causing disease on Juglans regia from Brazil (Mendez et al., 1998). No other species are reported to be hosts, or susceptible to infection by F. effusum.