Acknowledgements
4.5 Breeding Objectives
General objectives
Breeding objectives vary from region to region, depending on the specifi c trait(s) for which improvement is sought. However, they can be broadly gen- eralized to consist of the development of cultivars with: (i) regular bearing;
(ii) dwarf tree habit; (iii) precocity; (iv) attractive, good sized (300–500 g), good quality fruits (appealing fl avour and fi rm fl esh without fi bres); (v) resistance to major diseases and pests; (vi) freedom from physiological dis- orders; and (vii) good shipping qualities and shelf life. While it would be hard to combine all these characteristics within a relatively short time, espe- cially resistance to all major diseases and pests, all of these characteristics are basic for commercial success.
C.P.A. Iyer and R.J. Schnell 76
With regard to the improvement of rootstocks by breeding, the main desirable features are: (i) polyembryony; (ii) dwarfi ng; (iii) tolerance of adverse soil conditions (high pH, calcareous soil, etc.); and (iv) good scion- compatibility.
Specifi c objectives
In addition to improving general characters such as yield and quality, breed- ing has also been undertaken for certain specifi c purposes.
Dwarfness
Because of the obvious benefi ts of comparatively dwarf trees for orchard man- agement and fruit quality, attempts have been focused on obtaining hybrids with a dwarf tree framework. Breeding for dwarfness is important in mango, since a consistent dwarfi ng effect of any rootstock has not been established to date. The Indian cultivars that could be useful as a source for dwarfness include ‘Kerla Dwarf’, ‘Janardan Pasand’, ‘Manjeera’, ‘Amrapali’, ‘Creeping’
(Iyer and Subramanyam, 1986) and ‘Nileswar Dwarf’ (Singh, 1990).
Regular bearing
The causes of irregular bearing vary from region to region. In general, the main reason for alternate bearing, particularly in subtropical India, is the lack of initiation of vegetative growth soon after fruiting. However, two cul- tivars, ‘Neelum’ and ‘Bangalora’ (‘Totapuri’), which are regular bearers, have been extensively used as either of the parents in a hybridization programme to transfer the regular bearing habit to hybrids. ‘Neelum’ has been observed to be a good combiner and has contributed to the evolution of many regular- bearing Indian hybrid cultivars. However, ‘Bangalora’ is not a suitable par- ent since the hybrids possess very prominent beaks and their fruit quality is invariably poor. The regular bearing Florida cultivars (i.e. ‘Tommy Atkins’,
‘Keitt’, etc.) also have potential as parents.
Fruit colour
Most of the commercial cultivars in South-east Asia possess green skin.
Efforts are underway to produce new hybrid cultivars that retain the good qualities of these fruits together with attractive skin colour, so that they will occupy a better position in international trade. Since good skin colour has been shown to be transmissible to hybrids from suitably coloured parental cultivars, a number of cultivars with coloured skin are being used for hybrid- ization. In general, the attractively coloured Florida cultivars have been found to be suitable parents. In addition, there are several Indian cultivars (e.g. ‘Janardan Pasand’, ‘Suvarnarekha’, etc.) that would be suitable for use as parents for this purpose.
In Florida, the skin colour of the mango is an important factor and red skin is considered essential for mangoes shipped to northern markets. In the past, the evaluation of mango colour has been subjective and based on visual
Breeding and Genetics 77
ratings. Large errors are associated with these types of ratings, which makes evaluation based on fruit colour diffi cult. Ayala-Silva et al. (2005) used a colo- rimeter to quantify fruit colour, quality and differentiation among cultivars.
Mango colour was measured with a Minolta Chroma Meter CR-400 (Osaka, Japan) portable tristimulus colorimeter and fruit chromaticity was recorded in Commission Internationale de l’Eclairage (CIE) L*, a* and b* colour space coordinates. In this system of colour representation the values L*, a* and b* describe a uniform three-dimensional CIE colour space. If the L*, a* and b* coordinates are known, then the colour is not only described, but also located in quantifi able space. Maternal half-sib families (MHS) of the mango culti- vars, ‘Keitt’, ‘Tommy Atkins’, ‘Tyler Premier’, ‘Mamita’, ‘White Alfonso’ and
‘Sandersha’ were evaluated along with two parental check clones, ‘Tommy Atkins’ and ‘Keitt’. Signifi cant differences were found for each of the L*, a* and b* colour space coordinates. Further work is underway to estimate the herita- bility of these traits to estimate their usefulness for breeding and selection.
Disease resistance
MANGO MALFORMATION. Although no breeding work has been reported that spe- cifi cally addresses disease or pest resistance/tolerance, cultivars are known to show varying degrees of susceptibility to biotic stress (see Ploetz and Freeman, Chapter 8, this volume ). Mango malformation, caused by Fusarium subglutin- ans, is a very serious disease that has threatened the very survival of the mango industry in many subtropical mango-growing regions. As there are no reliable cultural and chemical control measures available, breeding for resistance/tolerance has been attempted using ‘Bhadauran’ as the resistant parent; however, all of the F1 hybrids were susceptible to the disease (Sharma and Majumder, 1988a). In this respect, the observations of Ram et al. (1987) are very encouraging. Out of 102 cultivars screened, three of them, namely,
‘Bhydayam Dula’, ‘Samar Bahist Rampur’ and ‘Mian Sahib’, were free of malformation and could be tried as one of the parents in hybridization.
BACTERIAL CANKER. Bacterial canker is a serious problem with many cultivars.
The only cultivar possessing true resistance to canker is ‘Bombay Green’
(Prakash and Srivastava, 1987) and hence could be a potential gene donor.
ANTHRACNOSE. Anthracnose, caused by Colletotrichum gloeosporioides Penz., is the most widespread disease in all mango-growing countries, manifesting itself in blossom blight, peduncle blight, leaf spot, twig blight, wither tip, fruit russetting and fruit rot. ‘Tommy Atkins’ is moderately tolerant of anthracnose and coupled with its other desirable qualities (i.e., regular bearing, fruit colour, etc.) should be a good parent in breeding programmes. In addition, ‘Parish’
and ‘Fairchild’ have been reported to be relatively resistant (Yee, 1958).
POWDERY MILDEW. Powdery mildew caused by Oidium mangiferae Berthet, has been reported to cause heavy loss of crops in years when RH is very high and accompanied by cool nights during fl owering. Cultivar differences with respect to susceptibility are recognized, and ‘Pairi’ (‘Raspuri’) is highly susceptible.
C.P.A. Iyer and R.J. Schnell 78
Gupta (1976) has listed those cultivars that are most tolerant of this disease –
‘Neelum’, ‘Zardalu’, ‘Bangalora’, ‘Totapuri-Khurd’ and ‘Janardan Pasand’ – and hence could be valuable in breeding programmes.
PEST RESISTANCE. Considerable variation is also known to occur among mango cultivars with respect to their susceptibility to attack and injury by insect pests.
Although no resistant genotypes have been reported for the mango hopper (Idiocerus spp.), the insect has been observed to avoid colonizing open plant types where free movement of wind is possible, an observation that could be useful in selection. Although complete resistance is not known to either fruit fl y (Bactrocera spp.) or seed weevil (Stenochetus mangiferae), variation in the degree of susceptibility has been reported (Iyer, 1991). Rossetto et al. (2006) observed that resistance to fruit fl y is compatible with fruit quality and pro- ductivity and advocated that resistance to fruit fl y should be one of the objec- tives of all mango breeding programmes. Their results also indicated that the main factors for resistance of mangoes to fruit fl ies lie in the fruit peel and not in the fruit pulp.