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Species Page

pecan scab

Fusicladium effusum

Distribution

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Host plants / species affected

Main hosts

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Carya illinoinensis (pecan)

List of symptoms / signs

Fruit - lesions: on pods
Fruit - lesions: scab or pitting
Fruit - premature drop
Inflorescence - lesions; flecking; streaks (not Poaceae)
Leaves - abnormal colours
Leaves - abnormal leaf fall
Leaves - fungal growth
Leaves - necrotic areas
Stems - discoloration
Stems - discoloration of bark
Stems - mycelium present
Whole plant - discoloration

Symptoms

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).

Prevention and control

Regulatory Control

There are no specific regulatory control measures for F. effusum in the USA beyond the standard regulations pertaining to the import and export of plant material. Australia does not have the disease but has strict phytosanitary regulation on the import of material from other countries (Anon., 1984).

Cultural Control

No cultural control measures are particularly effective for managing pecan scab. However, ensuring that the trees in an orchard are adequately spaced to ensure good sunlight penetration and air-flow will help reduce the severity on susceptible cultivars (Cooper and Johnson, 1986). Planting susceptible cultivars in locations that are low-lying and wet should be avoided. Ensuring old shucks (husks) are removed from trees after harvest can also reduce sources of primary inoculum, although being a polycyclic disease this is probably of very limited value.

Biological Control

Although not strictly biological control, Bacillus mycoides is a microbial agent that is thought to induce a systemically acquired resistance response and significantly reduces the severity of pecan scab (Brenneman, 2009).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:


This information is part of a full datasheet available in the Crop Protection Compendium (CPC);www.cabi.org/cpc. For information on how to access the CPC, click here.

Impact

Pecan scab causes yield and nut quality losses on susceptible cultivars (Gottwald and Bertrand, 1988; Stevenson and Bertrand, 2001). Nut drop can occur and total yield loss is possible if the disease is severe (Hunter, 1983). The effect of infection is to reduce net photosynthetic and dark respiration rates of both fruit and foliage (Gottwald and Wood, 1985) reduce nut number, nut weight, percent oil, moisture and protein content (Gottwald and Bertrand, 1983). Apart from a direct impact on yield and quality, the disease affects foliage and can result in loss of photosynthetic area and leaf drop. Severe scab can induce alternate bearing, the single most important problem facing the pecan industry in North America (Smith and Weckler, 2011). Reports of the impact of scab on pecan are most numerous in North America, but reports of its occurrence in South America and South Africa suggest that it has the potential to cause epidemics in these geographic areas. Fungicides are used to reduce the impact (Gottwald and Bertrand, 1988) but these are costly.