Alternaria alternata pv. citri causing brown spot of Minneola tangelos
In Israel in November 1989, an unknown disease was observed in several orchards of Minneola tangelos (Citrus reticulata cv. Dancy x C. paradisi cv. Duncan), in the region of Kefar Yona (central coastal plain, a long established citrus-growing area). At the beginning of fruit ripening, sunken, dark brown spots (minute to 3-6 mm diameter, 1-3 mm deep) were observed on the rind of fruits. Infected fruits were scattered all over the trees and many dropped to the ground. Leaves presented brown necrotic areas (circular spots to irregular blighted areas) and on some spots the dark discoloration extended into the veins. Apices of some young shoots were completely defoliated. Stems had occasionally circular or elongated brown spots. The causal agent was isolated in culture and identified as a citrus pathotype of Alternaria alternata (Solel, 1991). This disease observed in Israel was thought to be similar to 'brown spot of Emperor mandarins' which was first reported in Australia in 1966 (Pegg, 1966) and to 'Alternaria brown spot' of Dancy tangerines, and of Minneola and Orlando tangelos which was then reported in Florida (US) in 1976 (Whiteside, 1976). It is now considered that Alternaria brown spot is caused by Alternaria alternata pv. citri, although there is discussion on the validity of pathovars for Alternaria alternata. In the literature, the disease has sometimes been attributed to Alternaria citri, but the latter causes quite other symptoms and has a different host range. Another disease caused 'leaf spot of Rough lemon' has also been reported in South Africa (1929) and Florida (US), the causal agent is an Alternaria species but it seems quite distinct from Alternaria alternata pv. citri (Whiteside, 1988).
- Host plants
A recent field survey was carried out in Israel (Solel ; Kimchi, 1997) and showed that Alternaria alternata pv. citri is common on Minneola tangelos, but can also cause typical foliar and fruit lesions on: Dancy and Ellendale mandarins, Murcott tangor (mandarin x sweet orange), Nova and Idith mandarin hybrids, Calamondin (mandarin x kumquat (Fortunella)), and Sunrise and Redblush grapefruits. However, it was not found on: Shamouti Washington Navel and Valencia sweet oranges, Marsh seedless grapefruit, Clementine (mandarin), ortanique (mandarin x sweet orange), oroblanco (grapefruit x pummelo) or Eureka lemon, which are common in citrus groves near Minneola tangelos in Israel. In Florida, Alternaria alternata pv. citri is mainly reported on Minneola tangelos, Dancy tangerines, and less frequently on Orlando tangelos, Murcott tangors and Lee tangerines.
- Geographical distribution
In addition to Australia, Israel (where it became widespread on susceptible hosts soon after its first appearance; Solel et al., 1997), and Florida (US), Alternaria alternata pv. citri also occurs in South Africa. It has been present in this country apparently since the early 1980s, and its presence in several major citrus producing areas is considered as a serious threat to the cultivation of susceptible cultivars (Swart et al., 1998). In Turkey in 1995, 2000 ha of diseased Minneola tangelo trees were observed in the Cukurova region. 90% of orchards in this region were affected but the disease severity depended upon location and control strategies applied in each orchard. The symptoms observed were typical of Alternaria alternata pv. citri described in Israel. Alternaria was consistently isolated from disease leaves and fruits. This was considered as the first report of Alternaria brown spot of Minneola tangelo in Turkey (Canihos et al., 1997). The geographical distribution can be summarized as follows:
Distribution: Australia (first report in 1966), Israel (in 1989), South Africa (at least since the early 1980s) Turkey (in 1995), USA (Florida, in 1976).
- Biology and control
The fungus overwinters in lesions on leaves and stems or on any surviving out-of-season fruit. Spores are air-borne and they are produced more abundantly on leaf lesions than on infected fruit. The most important factors influencing brown spot are climatic conditions, host phenology and fruit maturity. Young foliage, twigs and immature fruits are most susceptible to infection. Climatic conditions as extended, interrupted wet periods also enhance infection.
Control of Alternaria alternata pv. citri is difficult as symptoms develop rapidly and sporulation of the pathogen occurs soon after infection. Fungicides like copper products, mancozeb, procymidone and iprodione can be used, and several studies have been carried out to optimize treatment methods (Solel et al, 1997; Swart et al; 1998). However, it must be noted that resistance to iprodione has been observed in some orchards in Israel and Florida, where intensive treatments with this compound had been done (Solel et al., 1996). Integrated control methods are needed, and should include for example: removal of inoculum sources; avoidance of pruning at wrong time, overhead irrigation, and over fertilization; appropriate timing and dosage of fungicides applications.
Pictures of symptoms can be viewed on INTERNET:
http://gnv.ifas.ufl.edu/~fairsweb/images/ch/ch074p2.gif (on fruit)
http://gnv.ifas.ufl.edu/~fairsweb/images/ch/ch017p2.gif (on leaves)
http://gnv.ifas.ufl.edu/~fairsweb/images/ch/ch017p3.gif (on leaves)
Sources
Canihos, Y.; Erkilic, A.; Timmer, L.W. (1997) First report of Alternaria brown spot of Minneola tangelo in Turkey.
Plant Disease, 81(10), p 1214.
Pegg, K.G. (1966) Studies of a strain of Alternaria citri Pierce, the causal organism of brown spot of Emperor mandarin.
Queensland Journal of Agriculture and Animal Science, 23(1), 15-28.
Solel, Z. (1991) Alternaria brown spot on Minneola tangelos in Israel.
Plant Pathology, 40, 145- 147.
Solel, Z.; Kimchi, M. (1997) Susceptibility and resistance of citrus genotypes to Alternaria alternata pv. citri.
Journal of Phytopathology, 145(8-9), 389-391.
Solel, Z.; Oren, Y.; Kimchi, M. (1997) Control of Alternaria brown spot of Minneola tangelo with fungicides.
Crop Protection, 16(7), 659-664.
Solel, Z; Timmer, L.W.; Kimchi, M. (1996) Iprodione resistance of Alternaria alternata pv. citri from Minneola Tangelo in Israel and Florida.
Plant Disease, 80(3), 291-293.
Swart, S.H.; Wingfield, M.J.; Swart, W.J.; Schutte, G.C. (1998) Chemical control of Alternaria brown spot of Minneola tangelo in South Africa.
Annals of applied Biology, 133(1), 17-30).
Whiteside, J.O. (1976) A newly recorded Alternaria-induced brown spot disease on Dancy tangerines in Florida.
Plant Disease Reporter, 60(4), 326-329.
Whiteside, J.O. (1988) Alternaria leaf spot of rough lemon.
In: Compendium of citrus diseases (Ed. by Whiteside, J.O.; Garnsey, S.M.; Timmer, L.W.), p 8. APS, St. Paul, USA.