1.6 Sweetpotato production constraints
1.6.3 Alternaria leaf petiole and stem blight
1.6.3.1 Occurrence and incidence of Alternaria leaf petiole and stem blight of sweetpotato
Alternaria blight occurs in most of the major sweetpotato growing regions of the world. It has been reported in South America especially in Brazil where it has been considered endemic (Lopes and Boiteux, 1994) and in South East Asia (Lenné, 1991) and has for a long time been reported in Zimbabwe (Whiteside, 1966) and Nigeria (Arene and Nwankiti, 1978). It was recently reported in South Africa but no economic yield losses have so far been reported (Narayanin et al., 2010a). In sub-Saharan Africa, the disease is common within the tropics and has been a major production constraint in Ethiopia (van Bruggen, 1984), Kenya (Gatumbi et al., 1991; Skoglund et al., 1994; Anginyah et al., 2001) and Rwanda (Ndamage, 1988). In Uganda, it has been reported in all regions of the country with the highest incidence in the Central and South-western highland agro-ecologies and the lowest in the Northern warm region (Mwanga et al., 2007; Osiru et al., 2007a). In Kenya (Skoglund et al., 1994; Anginyah et al., 2001) and Rwanda (Ndamage, 1988), higher disease incidence was reported at higher altitude areas.
However, Osiru et al. (2007a) recorded high disease incidence at both mid-altitude areas around Lake Victoria and at high altitude in south-western Uganda. The differences in occurrence and distribution of the disease are attributed to climatic conditions which are favourable for pathogen infection and disease development (Osiru et al., 2007a). According to Rotem (1994), the optimum temperature range for Alternaria species infection is 25-28ºC and these are the prevalent temperatures in the Lake Victoria Crescent Zone. In other areas where the disease occurs at mid- to high altitude, incidence and lesion size increase with altitude and
relative humidity since leaf surface moisture is necessary for infection and sporulation (Ames et al., 1996).
From their survey, Osiru et al. (2007a) reported low incidences of the disease in Uganda both at district and regional levels and the disease was recorded on most of the landraces. Similarly, Rotem (1994) observed differences in disease incidences among genotypes. Similar results were reported by van Bruggen (1984) in Ethiopia, and Lopes and Boiteux (1994) in Brazil, Skoglund et al. (1994) and Anginyah et al. (2001) in Kenya, and Narayanin et al. (2010b) in South Africa. In Uganda, Kenya and Ethiopia, lower disease incidences were recorded in the landraces that were more locally adapted than the newly introduced cultivars. Among the cultivars released by the Ugandan National Sweetpotato Program before 2003, only five (Bwanjule, Sowola, NASPOT 3, NASPOT 5, and NASPOT 6) exhibited moderate to high field resistance to Alternaria blight and the rest were susceptible (Mwanga et al., 2007). The observed differences in reaction of the indigenous and introduced genotypes to the disease can be attributed to the differences in the genetic base whereby local genotypes exhibited higher disease resistance levels possibly due to their broader genetic base (Anginyah et al., 2001). In Uganda, however, Osiru et al. (2007a) further noted considerable differences in reaction to the disease among the local genotypes, indicating that selection for disease resistance within these genotypes is also possible. Owing to the increasing incidence of the disease, and the lack of genotypes that are resistant, these observed differences in both local and introduced genotypes provide a basis for breeding for improved resistance since host plant resistance is the key to disease management in subsistence agriculture and in low value crops like sweetpotato.
1.6.3.2 Causal organism(s) of Alternaria blight of sweetpotato
Alternaria blight of sweetpotato is caused by a fungus of the genus Alternaria but the species differ from site to site. The pathogen identified as Alternaria capsci-annui was first reported in India (Sivaprakasam et al., 1977), and A. alternata has been reported in Papua New Guinea (Lenné, 1991), while A. bataticola has been reported in Brazil (Lopes and Boiteux, 1994). In sub-Saharan Africa, Alternaria blight caused by A. tax sp. (IV) has been reported in Ethiopia (van Bruggen, 1984), A. solani in Burundi and Rwanda (Ndamage, 1988), A. bataticola and A.
alternata in Kenya (Anginyah et al., 2001) and Uganda (Osiru et al., 2007a; Osiru et al., 2008;
Osiru et al., 2009). In Uganda, the incidence of A. bataticola was higher than of A. alternata in samples collected from across the country, thus A. bataticola is the most important species of the two (Osiru et al., 2007a; Mwanga et al., 2011). Identification and description of A. bataticola in Uganda was done by Osiru et al. (2008).
1.6.3.3 Morphology of the Alternaria blight pathogens, Alternaria bataticola and Alternaria alternata
Owing to the similar appearance of the isolates of A. bataticola and A. alternata, characterisation is done based on colony appearance or morphology and conidia shape. The morphological characteristics considered are the number of longitudinal and vertical septae, conidiophores as well as conidia length and shape (Osiru et al., 2008).
According to the International Mycological Institute description (David, 1991), A. bataticola (Ikata ex W. Yamamot) is characterised by mycelia that are “fuscous brown to almost hyaline, smooth-walled but occasionally rough-walled, septate, branched, 6-8 µm in diameter and 10- 30 µm in length”. On potato dextrose agar, the colonies are grey-green with a large amount of fluffy pale green aerial mycelium. The conidiophores are “single or in bundles, unbranched, erect or slightly curved, two to seven septate, pale brown to fuscous-brown”. In culture, the conidia arise as short side-branches on the main mycelium, unbranched, with one or a few conidiogenous loci. According to Osiru et al. (2008), “The conidia are solitary, elongate- obclavate, muriform, transversely five to eight septate, longitudinally zero to eight septate, pale to fuscous-brown, and smooth walled. The dimensions of the conidia are 69 (34-160) x 24 (15- 42) µm. The conidial beaks are long, filiform, colourless to pale brown, septate, and often branched with an average dimension of 8 (4-12) x 71 (32-129) µm”. However, some exceptions have been reported in Brazil where some isolates differed from the conventional description, and the conidia did not form branching beaks. These differences suggest that Alternaria of sweetpotato may be a complex disease across the world whose morphology can vary depending on site and prevailing environmental conditions.
Alternaria alternata differs in conidia shape and size from A. bataticola. “The conidia are brown, ellipsoidal, 20-60 x 15 µm (average 39 x 10.3) in size, short beaks, with two to six transverse septa, zero to four longitudinal septa and are catenulate at the apex of the conidiophores”
(Anginyah et al., 2001). The conidia are small pigmented, with short beaks, and borne on chains (Rotem, 1994).
1.6.3.4 Symptoms of Alternaria blight disease of sweetpotato
The common symptom of this disease is the formation of characteristic lesions. The lesions begin as small tan spots with light coloured centres that may enlarge up to several centimetres in diameter with concentric rings (Stathers et al., 2005). In carrot (Daucus carota L.), the ridge is slightly raised and thickened usually with concentric rings. Under favourable conditions, the lesions increase in number, expand and eventually coalesce and the affected leaf may shrivel and die. Large lesions on the petiole may also girdle and kill the leaf. Sometimes the infection originates on the leaf margin and progresses down the vein, petiole and stem (Gugino et al.,
2004). Manifestation of this disease varies with geographical location and several authors have described the symptoms differently depending on their location. According to Sivaprakasam et al. (1977), in India “the infected leaves show presence of dark brown to black irregular or more or less circular dead areas upon the leaves, which usually show concentric rings”. In a severe infection, a number of spots may coalesce to form large patches and the leaves get completely blighted and drop off prematurely. Furthermore, van Bruggen (1984) in Ethiopia described the symptoms as, “small, grey to black, oval lesions with a lighter centre on the stems and petioles”.
These symptoms are sometimes visible on the veins commonly on the lower side of the leaves.
Under humid weather conditions, the lesions on the stems enlarge into black areas and eventually become girdled and the leaves above the infected areas dry out. In dry conditions, the lesions become bleached. Skoglund et al. (1994) described the symptoms as “blackened lesions that occur on stems and petioles and later enlarge and coalesce until stems are girdled and killed”. Lesions occasionally occur on leaves and severe defoliation takes place especially on older vines. Stem blight manifests itself as stem necrosis and dieback is normally severe during the wet season and the soil beneath the diseased vines is carpeted with blackened leaf debris (Stathers et al., 2005; Osiru et al., 2008) (Figure 1.1).
Figure 1.1 Sweetpotato infected with Alternaria leaf petiole and stem blight at the National Crops Resources Research Institute (NaCRRI), Namulonge, Uganda
1.6.3.5 Ecology and epidemiology of Alternaria leaf petiole and stem blight disease
Alternaria blight can be differentiated from other leaf spots and blights because of its severity.
No other foliage diseases, with the exception of leaf and stem scab caused by Elsinoe batatas, have been reported to be so destructive. Alternaria blight is frequently observed at various stages of growth of the sweetpotato crop and under severe infection the soil under the diseased vines is carpeted with blackened leaf debris (Stathers et al., 2005).
As temperatures increase, the duration of leaf wetness required for infection to occur decreases. Infections can occur within 8 to 12 h at temperatures of 16-25ºC. Such lower
experienced (Stathers et al., 2005). The fungus sporulates readily on dead necrotic tissue and spores germinate readily in water droplets and dew. The disease is spread through infected planting material, wind, splashing rain, water and air currents (Skoglund et al., 1994; Mwanga et al., 2001; Gugino et al., 2004). The fungus survives as spores in plant debris and on volunteer plants (Stathers et al., 2005).
1.6.3.6 Management of Alternaria leaf petiole and stem blight
Healthy planting materials, field sanitation where infected crop materials are destroyed and burnt, crop rotation, and host-plant resistance are the common control measures at present.
Fungicides are effective but not widely used by resource poor farmers because they are expensive. Some resistant genotypes have been identified. For example in Uganda, cultivar Tanzania is resistant and is consequently grown throughout the country (Osiru et al., 2007a).