The influence of growth habit on the physiological response to ascochyta disease in broadbean (Vicia faba L.) and field pea (Pisum sativum L.) crops

Little is known about the influence of growth habit on the response to disease. The response may differ between determinate and indeterminate cultivars depending on how the disease affects the underlying radiation interception (RI), radiation use efficiency (RUE), and photosynthesis. An examination of the disease/crop growth habit interactions may provide a physiological insight into how crops respond to disease. The experiments were conducted using broadbeans (Vicia faba L.) and field peas (Pisum sativum L.) as the model crops and ascochyta blight (Ascochyta spp.) as the disease. In 1996/97, two broadbean plant types, the determinate 'Janet' and the weakly indeterminate 'Evergreen', and two field pea plant types, the determinate 'Beacon' and the indeterminate 'Whero' were examined under five disease intensities. In 1997/98, 'Beacon' and 'Whero' were examined under five disease intensities and three plant populations, while in 1998/99, 'Beacon' and 'Whero' were examined under two disease intensities and three canopy positions. Ascochyta blight infection was induced by inoculating plants with spore suspensions of Ascochyta spp. This study showed that both total dry matter (TDM) production and seed yield were affected by growth habit and disease. Averaged over both seasons, TDM and seed yield for 'Whero' were 33% and 8% greater than for 'Beacon' due to greater RI, RUE, and higher crop growth rates. Radiation interception was 23% and 20% greater for 'Whero' than for 'Beacon' in 1996/97 and 1997/98 respectively due to greater green leaf area index (GLAI) and area under the green leaf area index curve (AUGLAIC). Radiation use efficiency was greater for 'Whero' than for 'Beacon' despite 'Beacon' having a greater net photosynthetic rate than 'Whero' and may be attributed to differences in light saturation. The influence of crop growth habit on TDM production and seed yield was not consistent between broadbeans and field peas, with no significant differences in TDM between the broadbean plant types and 'Janet' out-yielded 'Evergreen'. Total dry matter production was similar for 'Janet' and 'Evergreen', while seed yield was greater for 'Janet' than for 'Evergreen' due to greater numbers of seeds per pod and a higher thousand seed weight. Radiation use efficiency over the growing season (RUES) was considered a more appropriate method of calculating RUE than RUE at final harvest (RUEF). Ascochyta blight reduced TDM production and seed yield by 25% and 27% in both seasons due to reductions in both RI and RUE. Radiation interception was reduced by 12% with disease in both seasons due to reduced GLAI and AUGLAIC. Radiation use efficiency was reduced by 18% (averaged over both RUEF and RUES, and both seasons) with disease and was associated with a 12% and 11% reduction in net photosynthetic rate in 1997/98 and 1998/99 respectively. The reduction in net photosynthetic rate with disease was influenced by canopy position, with disease reductions greater at lower canopy positions. The TDM and yield response to disease was influenced by growth habit but was not consistent between seasons. With the exception of pod profile, growth habit did not influence the response to disease for any other measurements. This study showed that TDM production and seed yield were affected by plant population, increasing by 22% from 50 plants m⁻² to 200 plants m⁻² due to a 27% increase in RI because of greater GLAI and AUGLAIC. RUE was not affected by plant population.