First report of Colletotrichum gloeosporioides causing withertip on twigs and tear stain on fruit of citrus in Morocco

Citrus trees (Citrus sinensis L. Osbeck) with symptoms resembling withertip on twigs and tear stain on fruits were observed in Morocco. Lime (Citrus aurantiifolia) was not affected. The dieback often progressed slowly and caused leaves to wilt, turn yellow and drop off. Twigs and branches appeared to have been scorched by fire (Figs. 1 and 2). When twigs were dry, minute brown-to-black, slightly raised, clumped pustules were observed. Under humid conditions, a pink slimy material appeared on dead bark and twigs. This symptom has also been observed on seedlings of citrus rootstocks in greenhouses in Morocco. Affected fruit showed tear stain symptoms (Fig. 3), as described by Fawcett (1936). The symptoms were superficial on the unbroken peel and formed dull red to dull reddish-green streaks or bands down the fruit. In some cases, these covered a large percentage of the fruit surface. On late season, overripe fruit, the streaks sometimes became brownish in colour, resembling rust mite damage.

In order to clarify the aetiology of this disease, symptomatic twigs and fruit were collected from a grove in the Gharb region. Affected twigs and fruit were washed, swabbed with ethanol and dried. Twig sections (10 cm) were placed on moist paper in a Petri plate. Pieces of peel, 5 mm in diameter, were plated on potato dextrose agar (PDA) in alternating light and dark at 25°C. Pink slimy masses developed from the black spots observed on the dead twigs (Fig. 4) and from incubated fruit peel. After plating onto PDA, a white mycelial growth was observed, along with the production of numerous salmon pink spore masses without setae and conidia that have both ends rounded (Fig. 5) The growth medium turned orange (Fig. 6). Growth of these isolates was almost completely inhibited on media containing 1 m g/ml of benomyl (Fig. 7) which is characteristic of Colletotrichum gloeosporioides (Peres, et. al, 2003).

Both C. gloeosporioides and C. acutatum have been reported on citrus (Timmer et al., 2000). However, C. acutatum only causes postbloom fruit drop of sweet orange and anthracnose of lime. Also this species produces conidia with at least one end fusiforme and all isolates should be inhibited only by ~50% on media with 1 m g/ml of benomyl (Peres et al., 2003). Only C. gloeosporioides was reported to cause withertip and tear stain of citrus (Klotz, 1961). The data provided confirms that this is the pathogen causing these symptoms on Citrus sinensis in Morocco. Pathogenicity was confirmed using Koch's postulates, with inoculations made on excised twigs, intact plants of Troyer citrange (Poncirus trifoliata x Citrus sinensis) and on fruit, using isolated spores. After incubation, tear stain and withertip symptoms developed on fruit and excised twigs/seedlings respectively. C. gloeosporioides was reisolated from excised twigs, fruits and plants. This is the first report of withertip and tear stain caused by C. gloeosporioides in Morocco.