Breeding of Psidium Species for Root Knot Nematode Resistance in Malaysia

The root knot nematode (Meloidogyne incognita) was reported to cause severe damage in guava cultivation in Malaysia. Several long term approaches were taken to find sustainable control for the root knot nematode through breeding. The approaches include protocol development to screen nematode resistance in Psidium species, genetic resource establishment in the field, screening of accessions against the nematode, identification of resistant rootstock and hybridization of resistant accessions with commercial cultivars. Protocol developed was used to screen more than twenty accessions against the nematode, and three accessions were found to be potential. These resistant accessions will be further evaluated for rootstocks. Hybridization among Psidium species was initiated to study the genetic compatibility among the species. Thirteen parental combinations were successfully hybridized and more than 2000 F1 interspecific hybrid seeds were sown and will be further evaluated for resistance to root knot nematodes. INTRODUCTION Guava is one of the fruit types that were given priority in commercial cultivation in Malaysia. The area planted with guava increased from 856 ha in 1986 to 1809.2 ha in 2000. Production of guava in 2000 was 11,674 mt for fresh consumption at value of RM11 million at the price of RM 1,000/mt (Table 1). This was generated from production area of 1,097 ha at the yield of 14.4 mt/ha for fresh (Anon, 2003). Expansion of guava cultivation is good because the crop was included in one of the priorities fruits under cultivation policy of Ministry of Agriculture especially in the Malaysian Third National Agriculture Policy. The most serious problem facing the guava industry in Malaysia is the root-knot nematode (Meloidogyne incognita). The attack was widespread across the country and all commonly grown guava varieties were reported to be susceptible (Abdul Karim and Yuen, 1994). Abdul Karim et al. (2002) screened 28 nematode-free seedlings of guava cultivar and showed that nematode gave high root galling index and high death rate for the guava cultivars evaluated except in red strawberry guava (P. littorale var. longipes) which showed apparent tolerance. Cuadra and Quincosa (1982) who screened 5 Psidium species in Cuba found that Costa Rican guava (P. friedrichsthalianum) was highly resistant to Meloidogyne spp. In nematode study, resistance plant is defined as plants that cannot prevent the parasite’s entrance but is able to prevent, restrict or retard its development. Resistance reactions are manifested when some factors in the host upsets the relationship between the host and nematode. Resistance is expressed by larvae failing to develop into adults, prolongation of the developmental cycle, larva death with root tissue, alteration of sex ratio toward maleness, or larva egress soon after penetration (Fassuliotis, 1979). In this work, the product is guava plants that can withstand the ravages of nematode attack, to enable them to grow and produce well. Previous works done by Abdul Karim et al. (2002) shows that local accessions have potential resistance to the root knot nematode and detailed studies have to be done to identify accessions systematically. The accessions can be multiplied and used as rootstock with 61 Proc. I IS on Guava Eds. G. Singh et al. Acta Hort. 735, ISHS 2007 commercial clones and can also be used in breeding programme with specific objectives. The breeding objectives of guava improvement programme in Malaysia can be summarised as development of guava plants with resistance to nematode, high yielding especially seedless varieties, flower and fruiting manipulation to optimize and continuous off season production and also to improve production system with less use of paper bags to reduce labour cost. This paper will report some of the progress made in long term approach to find sustainable control for this nematode through breeding approach. Various stages that were taken to achieve these objectives consist of: (i) Development of protocol to screen nematode resistance in guava species. (ii). Establishment of genetic resources materials in the field. (iii). Screening a range of Psidium species against the nematode. (iv) Using promising resistance as rootstock. (v) Hybridisation using accessions identified as resistance as parent with commercial clones. MATERIAL AND METHODS There are several stages in these long term programme of breeding guava for resistance to root knot nematode in Malaysia. Nematode Screening Procedure The detail screening protocol was developed and described by Abdul Rahman et al. (2005). Guava seeds were germinated in germinating tray. Sterilised top soil was filled in the polybag (17 cm × 7 cm). After 60 days, the seedlings were transplanted into polybag. Seedlings to be screened were prepared in four replicates. Once the plants were well-established, it was inoculated in each polybag with 5000 eggs of a known species of Melodogyne incognita. After around 60 days, the test was terminated by harvesting the plant. Plant damage was assessed for galls index by observation and counted the total eggs extracted from egg masses in the roots of the plant as in Table 2 (Taylor and Sasser, 1978). Nematode reproduction was assessed and host efficiency was calculated using Oostenbrink’s reproduction factor: R = Pf/Pi, where Pi was the initial inoculum levels (5000 eggs) and Pf was the final egg formed in the root. Canto-Saenz (1983) host suitability designations were assigned on the basis of the scale proposed by Oostenbrink (1966) as in Table 3 and Table 4. Genetic Resources Collection, Conservation and Evaluation 1. Long Term Genetic Conservation. A total of 24 accessions were already established in germplasm plot at MARDI Serdang. These consist of Department of Agriculture, Malaysia registered clones as well as selected clones by individual farmers, nurseryman and include other than P. gujava species. This collection will become a genepool based for future research with various objectives including selection for root knot nematode. 2. Genetic Collection and Evaluation for Root Knot Nematode. A total of 180 accessions were collected from local and overseas. In this collection, locally natural grown P. guajava and others Psidium species fruits were collected and germinated in the glasshouse. Those that were germinated were given selection pressure to select against root knot nematode using method developed by Abdul Rahman et al. (2005). Those accessions that showed potential resistance were planted in the germplasm plot for long term conservation and evaluation. Selection of Accessions for Rootstock The potential accessions that showed tolerance or resistance to root knot nematode were multiplied and raised in the glasshouse for rootstock selection. The selected seedlings were used as rootstock and will be grafted with scion from established commercials clones. All grafted seedlings were evaluated in nematode hot spot area in the field.