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OSU231, PANNAR’s A076 and Embu11 from the Kenya Agricultural Research Institute (KARI) (ISAAA, 1999). In the current study for the development of Mozambican MSV resistant hybrids, lines better adapted to the tropical lowland conditions other than the above mentioned lines were selected. The parental inbred lines for each study were chosen based on their diversity in disease resistance and adaptation to the environment.
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et al., 1998; Jeger et al., 1998). The conidia produced by P. sorghi on erect conidiophores which grow out through leaf stomata, are copiously produced in thin- walled structures that allow for the rapid polycyclic increase and spread of an epidemic within a season (Jeger et al., 1998; Adenle and Cardwell, 2000).
1.3.2 Geographic distribution of downy mildew disease
Downy mildew disease was first reported in India in 1907 and is now widely distributed in Asia and Africa (Frederiksen and Renfro, 1977; Bigirwa et al., 2000). In Asia, it is considered as one of the most destructive diseases of maize causing yield losses of up to 50%, thus making it a top priority biotic stress factor of maize (Pingali and Pandey, 2001; George et al., 2003). In Africa, DM disease outbreaks have been reported from Uganda, Mozambique and the Democratic Republic of the Congo (Ajala et al., 2003).
In Nigeria the disease has been widespread because of continuous cultivation of maize throughout the year and is a serious constraint to maize production, especially in the forest zone (Kim et al., 1994). In Mozambique, P. sorghi has been identified only on maize and not in sorghum (Plumb-Dhindsa and Mondjane, 1984).
1.3.3 Symptoms of downy mildew
Maize plants infected by P. sorghi at the seedling stage have a characteristic stunted appearance and may die prematurely approximately four weeks after infection (Jeger et al., 1998; Ajala et al., 2003). Leaves of older plants display the characteristic chlorotic streaking that begins at the base of the leaf with a clearly defined margin between diseased and healthy tissue (Fig. 1.2A). Infected plants have leaves that are narrower and more erect than healthy leaves (Craig and Frederiksen, 1983; Jeger et al., 1998;
Ajala et al., 2003).Further, these plants may not seed, thus cobs fail to form and the tassel is replaced by a mass of twisted leaves exhibiting ‘bushy’ growth, hence the top is usually referred to as “crazy top” (Ajala et al., 2003).
20 Figure 1.2: Downy mildew disease. A: Close-up of downy mildew disease symptoms of maize infected with P. sorghi, showing the characteristic white striping of leaves, which always includes the base; B: Comparison of downy mildew (DM) resistant and DM susceptible maize plants; on the right is maize with DM incidence approaching 100%, there is also stunting induced. DM resistant variety on the left is thriving. Photo by: Nothando F. Mafu, PMB, UKZN. Taken at IIAM Chokwe, Mozambique. Date: 13/03/11.
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1.3.4 Damage caused by downy mildew disease
Downy mildew is one of the most destructive maize diseases in Nigeria and Mozambique (Pingali and Pandey, 2001; Ajala et al., 2003). In Mozambique, it causes crop losses in the lowland areas in the central and southern regions. The losses have resulted in some extensively grown varieties being withdrawn from the market during 2003-2006 due to their susceptibility to DM disease (Mariote, 2007). High relative humidity of about 90%, temperature ranges from 20-25°C and rainfall mostly favours disease development (Amusa and Iken, 2004). Yield losses as a result of DM infection have been reported to range from 10-100% (Fig.1.2B) (Gowda et al., 1987). Therefore, the disease requires effective control strategies to minimise economic damage.
1.3.5 Control methods of downy mildew disease 1.3.5.1 Cultural control
Cultural control methods can be used to manage DM disease in maize. These involve planting in well drained soils to reduce oospore growth, burying any infected crop debris to reduce inoculum sources and simultaneously cultivating maize with the alternative hosts (e.g. sorghum) of downy mildew disease (Bigirwa et al., 2000). The impact of diseases depends on the age of the plant; therefore adjusting planting times such that crops can escape high disease pressures can also be used as a means to control DM disease (Gilbert, 2002). For example, Frederiksen and Renfro (1977) reported that young plants outgrew DM disease when they were not infected at the seedling stage.
1.3.5.2 Chemical control
Chemical control involves the use of fungicides (George et al., 2003). While it has been effective, fungicides are not readily available in remote areas of Mozambique (Mariote, 2007). In addition, an emerging problem as a result of intensive use of fungicides is the build-up of chemical resistance in pathogens making some fungicides inefficient
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(Perchepied et al., 2005). As with MSVD control, resistance breeding is perceived as the best solution for the control of DM disease (George et al., 2003; Perchepied et al., 2005). This strategy is advocated in the current study.
1.3.5.3 Use of downy mildew-resistant maize genotypes
In West Africa, breeding for downy mildew resistance (DMR) in maize started in the mid-1970s as a collaborative effort between the Nigerian National Maize Programme and the IITA. The effort resulted in the development of early and late maturing open- pollinated resistant varieties released for cultivation in the mid-1980s (Ajala et al., 2003). The use of resistant varieties is proving to be more cost-effective and an environmentally safe alternative for controlling DM diseases (George et al., 2003;
Perchepied et al., 2005).