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Selection of papaya ringspot virus-resistant plants

 

Sistema de Investigación del Golfo de México
www.ecologia.edu.mx/sigolfo

Raúl Mosqueda V.
Enrique N. Becerra
Gregorio Arellano
Fidel De Los Santos
Xóchitl Rosas
Angel Villegas

 

Introduction

Papaya growing (Carica papaya) is affected by 15 different diseases in regions where this variety is grown. In Mexico, four of these diseases have been identified. Economically, papaya ringspot virus is the most harmful (VMAP). In Mexico, this virus was identified in 1975. It was discovered that papaya ringspot virus had caused damage varying between 37% to 100% of crops. In 1987 and 1994, this virus continued causing problems for papaya growers.

From 1980 to 1991, INIFAP researchers carried out some studies in Veracruz in which recent viruses, vector insects (their populating fluctuation through time), and disease epidemiology were identified, as well as studies of growing and chemical control demonstrated their efficacy. Since 1987, University researchers have also made important contributions to the papaya ringspot virus control in Veracruz.

The genetic resistance to this disease is one of the factors in which there have been no advances in Mexico.

Until a few years ago, when some papaya varieties, from other countries, were grown for trade production (such as Maradol, Solo and recently Red Lady variety) only some domestic varieties, having appropriate fruit and plant variability, were grown in Mexico.

For this reason, a genetic improvement program was carried out in order to reach a polycross-breeding. Between 10 different varieties were used as parent plants to achieve variability in papaya ringspot virus resistance and moreover to combine certain advantageous features of the plant and the fruit.

In Campo Experimental Cotaxtla, (Cotlaxtla Research Farm) we have already grown the second generation of polycross-breeding segregated growings and their parent plants. After some research was conducted to accelerate the process of genetic improvement, we can conclude that one of the options is to select individually those varieties which show papaya ringspot virus resistance, high fruit productivity (whose fruit detachment rate is 1.66 fruits per week) and an appropriate fruit quality (average weight between 1.8 to 2.5 kilograms, soluble solids in more than 10% and flesh thickness close to 1.0 kilograms/cm2). Another very important factor to consider is that the clonation in vitro of these varieties was achieved to improve their characteristics by applying a technique based on a study (Litz and Conover 1978). The following were aims of this project.

 

Research Procedures

Farm

From October 1996 to January 1997, both papaya ringspot virus impact and severity and the total number of fruits produced by this plant in 12 weeks of fructification were analyzed. In accordance with these two criteria, 76 plants from a total number of 1300 were selected. Because of some diseases, 30 plants died. Forty-six remaining plants were analyzed by using different selection criteria such as fruit weight, soluble solids and flesh thickness. Finally, as a result only 19 plants were accepted. This means a very strict selection process of 1.5%. In early June 1997, some apical and lateral buds from 19 selected plants were sent to the tissue culture laboratory in Chapingo, Mexico.

Laboratory

Three hundred ninety-six meristems from 19 parent plants were sowed. During the period of elongation, there were some contamination problems caused by bacteria and fungi. These disease producers went on causing troubles in the medium. This type of problem is described by Litz and Conover (1978) as limitative and relevant for in vitro propagation of papaya. It was necessary to consult experts who identified these bacteria and recommended antibiotics to resolve this problem. In September and November 1997, growers needed to collect and sow new buds again to replace those lost in contamination.

Results

Farm

Those genetic materials which should be used as a resource of better characteristics in genetic papaya improvement were identified. Pinare and Clariflora showed, in impact and severity, a better papaya ringspot virus resistance. These varieties also showed a greater delay to the beginning of the infection and a lower rate of disease increase. Other results confirmed that Sunset variety stood out in greater number of fruits; Icu variety, in heavier fruit weight; Cariflora variety, in a higher content of soluble solids; finally, Red Maradol, in a better flesh thickness.

Laboratory

It was only possible to obtain calluses of 12 clones from 19 originally cultured in MS medium with 50 microM of benzyl adenine (BAP) and with 10 microM of Naphthalene Acid (ANA). Later, these clones were subcultured twice in 100% MS medium with 2microM BAP and with 0.5 microM ANA. Under these conditions, only four clones have been able to proliferate buds. Clearly, this indicates a distinguishing result of genotypes, which is a disadvantage because in the long run the medium will not be able to propagate outstanding clones in vitro. Successfully cultured clones have Hawaiian group papaya-like germoplasm. In order to achieve bud elongation, it was determined that 50% MS medium supplemented with Ag3 has been much better and specifically, PM 3-1-49 clone has 1.3 centimeter length buds, therefore it is considered appropriate to begin with rooting.

In this point of the research, it has been leading PM 3-1-49 clone into rooting in a 50% MS medium with 2 mg (AIB), in the presence and absence of light.

PM 3-1-49 clone parent-plant characteristics are:

• Papaya ringspot virus resistance
• Fruit productivity of 1.75 fruits per week
• Fruit medium weight of 1.950 kilograms.
• 10.4% soluble solid yellow flesh
• 0.70 Kg/cm2 flesh consistency If shoot is achieved, the presence of these characteristics will demonstrate that in vitro propagation was successful.

 

 

Bibliography

Andrade, H., E. García , A. Mora, D. Nieto, D. Téliz y J. Villanueva. 1994. Manejo integrado del virus de la mancha anular del papayo en Veracruz, México. XL Reunión Anual. Interamerican Soc. for Tropical Horticulture. Campeche, Camp. México. p. 175.

Becerra L., E.N. 1994. Manejo integrado del virus de la mancha anular del papayo. Revista de la Universidad Cristóbal Colón. 5(11):17-27. Veracruz, Ver. México.

De León M., J:R: y E. N. Becerra L. 1991. Estudio y control de la virosis en papayo. 4a. Reunión Anual del INIFAP en el edo. de Veracruz. Publ. esp. No. 8. Veracruz, Ver., México. p.54-60.

Litz, R.E., and R:A: Conover. 1975. In vitro propagation of papaya. HortScience, 13(3): 241-242.
Rodríguez E., J.G. 1994. Distribución de las virosis del papayo en México. Tesis M.C. Colegio de Postgraduados, Montecillo, Mex. 84 p.

Téliz D., A. Mora, D. Nieto, D. Gonsalves, E. García, L. Matheis y C. Avila. 1991. La mancha anular del papayo en México. Rev. Mex. de Fitopatología, 9:64-68.

 

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