The resistance and tolerance of different cocoa varieties to cocoa swollen‐shoot virus in Ghana

Abstract

SUMMARY The resistance and tolerance to infection with cocoa swollen‐shoot virus (virulent strain A) of forty‐five and thirty‐eight cocoa progenies respectively was estimated using a manual inoculation method. Resistance was estimated from the proportion of inoculated seedlings which developed symptoms, and tolerance from the severity of these symptoms during a 4‐month period. Resistance was found among Nanay, Iquitos and Scavina, all Upper Amazon types; and an inter‐Nanay cross was outstandingly resistant and offers scope for improvement by breeding. Some Amazon‐Amazon and Trinitario‐Amazon progenies snowed tolerance, especially those with Trinitario T9/21 as a parent. Some progenies seem sufficiently promising for field testing to select varieties suitable for planting in those areas in Ghana where swollen‐shoot disease is epidemic.

Resistance of cocoa to swollen‐shoot virus in Ghana. I. Field trials

“…Seed inoculation methods were used to select the parents and hybrids under test in trial A14 (Table 3) for both resistance and tolerance of infection (Legg & Kenten, 1970). This and a related tolerance trial assessed the field reaction to infection of material which at the time of planting showed the greatest potential for resistance and tolerance.…”

“…The remaining parents (Tables 3-9) were GA11, a Trinitario from Grande Anse, Haiti and West African Amelonado. in trial B5b because it was considered to be outstandingly resistant (Legg & Kenten, 1970). Two standards were T85/799 x Amelonado, which was one of the highest yielding hybrids known in Ghana, and a well tested inter-Upper Amazon hybrid, T85/799 x T79/501, which had consistently shown a low incidence of black pod disease (caused by Phytophthora palmivora) and good agronomic characteristics (Lockwood, 1976).…”

“…Immunity is unknown. Posnette & Todd (195 1) found that seedlings of Found's (1938) Iquitos Mixed Calabacillo (IMC) and Nanay (NA) collections from near the centre of diversity of cocoa in the upper reaches of the River Amazon were more resistant than Amelonado, a difference which was repeatedly demonstrated subsequently (Dale, 1 9 5 7~ b; Legg & Kenten, 1970). In field trials the rate of virus spread in hybrids derived from Upper Amazon material was one-third to a half of that in Amelonado and it was suggested that such resistance was of immediate practical value (Legg & Kenten,197 1).…”

Tolerance trials with young cocoa trees infected with cocoa swollen‐shoot virus

“…The trial also included the hybrid between A471537 (which yielded well as a healthy seedling and gave evidence of tolerance in seed inoculation tests) and Elb/83 (which yielded well as a naturally infected tree). The progenies in trials E l 1 and E l 3 were selected on the basis of mild leaf symptoms in manual inoculation tests (Kenten, Legg & Bonney, 1969;Legg & Kenten, 1970). In both trials the standard progeny was Series IIB, a local-Trinitario x Upper Amazon hybrid being used by farmers.…”

“…However, Lockwood (1973) found that virus infection greatly reduced yields of progenies from tolerant parents (Glendinning, 1966). On the basis of the Nigerian work and the earlier Ghanaian results, Legg & Kenten (1970) used symptom severity in young seedlings to aid selection of tolerant hybrids for use in Ghana. Hybrids similar to those rated as tolerant of infection with either the Nigerian Egbeda or Offa-Igbo virus strains (Longworth, 1965) proved to be sensitive to the Ghanaian New Juaben strain of CSSV, but other progenies were found in which leaf symptoms were mild.…”