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Lesions on young leaves begin as minute water-soaked spots which subsequently evolve into amphigenous, creamy-yellowish or variously bright-coloured pustules. These grow as irregular, globose or conical excrescences which coalesce and extend mostly along the main veins to cover a large part of the leaf blade, particularly on the lower surface. The central area of these wart-like outgrowths is depressed and becomes drab, greyish and velvety when the fungus is fruiting. Old scab lesions have a rough surface, are dusky-coloured and become cracked and fissured. Affected leaves become stunted, malformed, wrinkled or puckered, with irregular torn margins. Defoliation often follows severe infections. Similar warty lesions and corky eruptions are formed on young twigs, tender shoots and stems of nursery plants which can grow bushy and stunted. Blossom pedicels and buttons may be attacked as well. Fruits are infected in the early stages of their development, grow misshapen and are subject to premature fall. On the rind of developed fruits, raised lesions are formed with different shape, size and colour according to the species and cultivar affected. They appear as scattered protuberances, conical projections or crater-like outgrowths, or they coalesce to give scabby patches or extensive areas of fine eruptions. Scab lesions do not extend into the albedo.
Citrus scab can be controlled using resistant cultivars (Ieki, 1982; Yoshida and Shichijo, 1984; Reddy et al., 1986) and by fungicide applications both in the nursery and in the orchard. Protectant fungicides may be applied (copper, ferbam, thiram, difenoconazole and chlorothalonil have been used), or systemic fungicides (benomyl, carbendazim) before flushing and after petal fall (see González, 1980; Rao, 1983; Reddy et al., 1983). Benomyl-tolerant strains of the pathogen have been found (Whiteside, 1980).
Crop sanitation, establishing citrus nurseries in dry areas or in greenhouses, and adoption of proper treatments, may help in production of rootstocks and budwood free from the pathogen. The usual procedures for importation of certified citrus planting material should be followed. For more information, see also Knorr (1977) and Roistacher et al. (1977).
Of the two citrus scab pathogens, E. fawcettii is the more widespread but E. australis is more economically significant as it attacks species of citrus which are more widely grown.
In the field, E. fawcettii affects lemons, mandarins, tangelos and grapefruits, whereas most cultivars of oranges and limes are less or not affected. The disease may be serious in the nursery on susceptible rootstocks such as sour oranges, rough lemons, Poncirus trifoliata and Citrus limonia. It may stunt seedlings or make them bushy and difficult to bud. Scabs are present, particularly on the young growth. Infested nursery stock is the primary means by which scab is introduced into new plantings. Severely infected fruits are scarred and distorted and consequently unmarketable. E. australis differs in only causing fruit scab, mainly on oranges and mandarins.
Citrus scab is widespread in areas where suitable conditions of temperature and rainfall or high humidity prevail (wet subtropics and cooler tropics). Elsewhere, it occurs when new flush and fruit set coincide with spells of relatively warm, humid weather. It is also favoured by local conditions such as damp, low-lying areas and dense, shaded citrus groves. Citrus scab is important only in areas where susceptible species or cultivars of citrus fruit are grown for the fresh market and where young plants or new growth develop under favourable conditions of temperature, moisture and shade. Losses largely depend on seasonal and local variations in weather. The disease is not a problem in areas with a limited annual rainfall (less than 1300 mm), long-lasting hot seasons (mean monthly temperature above 24°C) or a dry summer. In the Mediterranean region and, more generally, in citrus-growing areas with a dry climate (e.g. California and Arizona in the USA, where the disease has never become established) scab, even if present, is rare or unimportant.
