Colletotrichum acutatum (black spot of strawberry)

The spread of the disease is often so rapid that by the time symptoms are noticed, the crop is in serious danger. For strawberry, fruit and occasionally petiole rots may be noticed, with sunken, water-soaked spots enlarging to cover the whole fruit within 2-3 days, with dark-brown fruit bodies producing pink spore masses. For other crops such as anemone and celery, crown rots and leaf curl may be the principal symptoms. In pine seedlings, the developing leaves around the apical bud are affected, with small, brown lesions appearing and rapidly extending. Severe stunting is eventually caused as the uninfected tissue beneath the apex continues to develop.

The only serious research on control has been in connection with strawberry crops. Some success was reported in New Zealand by spraying with dichlofluanid and a captan-benomyl mixture (Cheah and Soteros, 1984), with various chemicals in Australia (Washington et al., 1992), and in South Africa with captan (van Zyl, 1985). Recently in the USA, studies showed that no acceptable fungicide is effective (Milholland, 1989). Fungicide-resistant strains of related species have been reported in the USA and Japan (Chikuo and Kobayashi, 1991; McInnes et al., 1992). There have been considerable efforts in the USA to develop resistant strawberry cultivars, but limited success has been achieved due to the presence of varied races within the species (Delp and Milholland, 1981; Smith, 1985; Smith and Black, 1990; McInnes et al., 1992). Gupton and Smith (1991) have suggested some potentially useful directions for further research.

In the UK, the disease is rare owing to strict quarantine controls and a policy of destroying affected crops and fumigating soil. McInnes et al. (1992) found that nursery material derived from tissue culture which was free from the related species C. fragariae and planted in isolated fields remained healthy, suggesting that careful selection of disease-free stock and soil sterilization in affected beds might be at least as effective as attempting chemical control.

In celery crops, Wright and Heaton (1991) found both a variation in cultivar susceptibility and amenability to chemical control of the disease. For anemone, disease incidence decreased with storage of corms (Doornik and Booden, 1990), and treatment by soaking with hot water proved effective (Doornik, 1990). Yearsley et al. (1988) found that dipping of tamarillos in imazalil and prochloraz reduced the incidence of postharvest disease caused by C. acutatum. However, dipping strawberry plants in hot water or fungicides did not eliminate the disease.

For pine, regular applications of prochloraz have been found to be effective, as has dichlofluanid (Vanner, 1990).

The disease is significant worldwide on strawberry (on which it is considered the second most important pathogen after Botrytis cinerea), and also on a few other crops such as anemones. The disease on pine may not now be so severe as in recent years, judging from the decline in research papers. Little detailed information on economic losses is available. In France, the disease has caused up to 80% losses of unsprayed strawberry crops, especially of ever-bearing cultivars (Denoyes and Baudry, 1991). Crops sprayed for B. cinerea control have suffered much less. In the UK, where the disease is statutorily notifiable, presence forces the burning of crops and fumigation of the soil.

Recent studies in Australia showed that C. acutatum caused losses of 25-50% in celery crops in Queensland (Wright and Heaton, 1991).