Black rot

Black rot, also called Thielaviopsis fruit rot, water blister, soft rot or water rot, is caused by the fungus C. paradoxa (De Seynes) Sacc. (syn. T. paradoxa (De Seyn.) Hohn (telemorph C. paradoxa (Dade) C. Moreau). The disease is a universal fresh-fruit problem but normally not a problem with processed fruit, because times from harvest to processing are too short for infection. The severity of the problem is dependent on the degree of bruising or wounding during harvesting and packing, the level of inoculum on the fruit and the storage temperature during transportation and marketing. Black rot does not occur in the field unless fruit is overripe or injured.

Black rot of the pineapple fruit is characterized by a soft watery rot, which usually starts at the point of detachment of the fruit (Fig. 9.25). Diseased tissue turns dark in the later stages of the disease because of the dark-coloured mycelium and chlamydospores.

Infection of the pineapple fruit occurs through wounds resulting from harvesting and postharvest handling. Susceptibility varies, with the 'Red Spanish' types being more resistant than 'Smooth Cayenne'. Under conditions of high humidity, conidia may readily be produced on pineapple residue and be disseminated by wind to the unharvested fruit. Inoculum levels on fruit at harvest vary according to the environmental

Fig. 9.25. Cross-section of fruit showing black rot caused by Chalara paradoxa at the broken peduncle and through a bruise on the side of the fruit.

conditions prior to harvest (Rohrbach and Schmitt, 1994). The high correlation between moisture (rainfall duration) prior to harvest and disease following harvest has resulted in the name 'water rot'. Infection occurs within 8-12 h following wounding. Refrigeration at 9°C during transportation will slow development of the disease, but, when fruit are returned to ambient temperatures, disease development will resume (Rohrbach and Phillips, 1990).

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