The effects of various morphological characters (frego bract, red plant body, okra leaf and nectarilessness) of the cotton plant on crop development, growth and performance as well as resistance to Heliothis spp. and plant bugs and the interactions between plant pest and beneficial species were investigated over four seasons between 1975 and 1979.
During the 1975-76 season, material from different germplasms (Deltapine 16, Deltapine smooth leaf, M8, M8 Superokra leaf, Namcala, Riverina poplar, Stoneville 7A, Stoneville 7A nectariless) were studied under unsprayed conditions.
Egg deposition among the cultivars was similar but larval abundance was reduced on M8-SOL and St-7AN.
M8-SOL was very susceptible to Xanthomonas damage resulting in severe boll loss and consequent seed-cotton yield.
St-7AN by virtue of the reduced Heliothis pressure on it had less boll damage and the highest seed-cotton yield.
These results were difficult to interpret on the basis of the characters (i.e. nectarilessness or superokra leaf) alone as they were confounded by background germplasm effects. The germplasm effects were apparent through familial associations.
In subsequent seasons (1976-78) near-isogenic F3 populations from Coker 201 background (Gossypium hirsutum. (L)) were studied under sprayed and unsprayed field conditions. Glasshouse trials were conducted to study earliness of maturity and seed-cotton yield potential in detail. Laboratory studies were also conducted into the effect of the characters on (i) Heliothis punctigera Wallengren growth and development when fed squares from the different cultivars and (ii) larval establishment on the plants.
Okra leaf maintained high fruiting and flowering rates with a resultant significant improvement in earliness of crop maturity. Frego was late in crop maturity.
Red plant was consistently more preferred for oviposition by Heliothis spp. but high egg/larval mortality was associated with this cultivar under the unsprayed field conditions. Red plant thus appeared tolerant to Heliothis infestations.
Frego was less preferred for Heliothis oviposition and egg/larval mortality was also increased on this cultivar. Consequently Heliothis damage to fruiting forms was reduced on this cultivar enabling it to set an earlier crop under the unsprayed field conditions. Plant bug damage, however, was higher on Frego but this was not economic.
The 'Composite' a cultivar that combined red plant colour, okra leaf, frego bract and nectarilessness was associated with (i) a significant delay in days before flowering, (ii) din increased susceptibility to plant bug damage and (iii) a depression in boll size and seed-cotton yield.
None of the cultivars appeared to have any direct influence on Heliothis survival on growth and development as observed under glasshouse or laboratory trials.
During the 1978-79 season cultivars with the characters, red plant colour, frego bract and okra leaf were studied in single trait and multiple trait combinations. Analyses based on phenotypic means and orthogonal comparisons showed:
ⅰ. the okra leaf gene (Lo) to induce earliness of crop maturity,
ⅱ. the red plant gene (Rl) reduced boll size and seed-cotton yield. It also induced a higher preference by Heliothis moths for oviposition, and incidence of parasitism from three species,
ⅲ. the frego bract gene was associated with lateness of maturity and induced higher levels of parasitism among Heliothis larvae.
ⅳ. frego bract and red plant colour was additive in increasing the level of parasitism,
ⅴ. frego bract and okra leaf improved earliness of maturity over the frego bract alone.
In a 'background and contained plot' experiment where all cultivars were planted in each others environment, the okra leaf character appeared to provide an environment which reduced the activity of de-fruiting insects and caused cultivars within this environment to produce more flowers than the normal leaf environment.
Whilst the frego bract character may afford some protection to the cotton plant in years of light infestation, none of the characters or combinations of them (frego bract included) would be able to protect the cotton plant in years of high infestation as in 1978-79 when Heliothis pressure was high no cultivar produced a crop.