Quantifying and Modelling the Mobilisation of Inoculum from Diseased Leaves and Infected Defoliated Tissues in Epidemics of Angular Leaf Spot of Bean

Abstract

Daily multiplication factor (number of daughter lesions per mother lesion per day) values were experimentally measured in four replications of a monocyclic experiment on angular leaf spot (ALS) of bean, where sources of inoculum were artificially established within a bean canopy, on the ground (defoliated infected leaves), or both. Daily multiplication factor of lesions in the canopy (DMFRc) was higher than that of infectious, defoliated tissues (DMFRd) in all replications. Both DMFRc and DMFRd were strongly reduced under dry compared to rainy conditions. Under rainy conditions for spore dispersal DMFRd was about two to three times smaller than DMFRc. Defoliated leaves may nevertheless represent a significant source of infection, depending on the amount of infectious tissues. Mother lesions within the canopy generated more daughter lesions in the medium (or lower) layers of the canopy than at its upper level (DMFRc higher at the medium and lower layers of a canopy), whereas DMFRd values seemed to decrease with height in the canopy. A mechanistic simulation model that combines host growth and disease-induced defoliation was designed to simulate the respective contributions of the two components of the dual inoculum source of a diseased canopy (infected foliage and defoliated infectious tissues), and varying infectious periods in both sources. Simulations suggest that higher DMFRc values have a large polycyclic effect on epidemics whereas that of DMFRd is small, and that large effects of the infectious period of lesions in the canopy are found when DMFRc is high. Simulations using experimentally measured DMFRc and DMFRd values indicated much stronger epidemics in rainy compared to dry conditions for spore dispersal, but disease persistence in the latter. The implications of considering a dual source of inoculum in the course of a polycyclic process are discussed with respect to epidemic thresholds.

Soybean and grapevine rusts accelerate the defoliation rates of host plants

“…This reduction is higher than that caused by disease severity and has great consequences on the partitioning of carbohydrates towards the different organs of the plant, reducing yield. Hence, to simulate partitioning of carbohydrates caused by both rusts, it is necessary to (i) estimate a parameter that describes how the rate of leaf drop is associated with disease severity and (ii) incorporate this parameter into the simulation model of the healthy host growth (Allorent et al., 2005; Willocquet et al., 2004). Defoliation is negatively correlated with yield in soybean due to the low amount of carbohydrates translocated and stored in the grains (Mueller et al., 2009).…”

Comparative epidemiology of grapevine and soybean rusts

“…pachyrhizi on their hosts is senescence acceleration, which is associated with early leaf fall and negatively related to grapevine and soybean yields. In order to simulate yield losses caused by Phakopsora rusts, it is necessary to estimate a parameter representing the rate of leaf fall that is related to disease severity and incorporate this parameter into the simulation model (Willocquet et al, 2004;Allorent et al, 2005).…”