Pathometric relationships reveal epidemiological processes involved in carrot cavity spot epidemics

Abstract

Carrot cavity spot (CCS) is one of the most important soilborne diseases affecting the carrot crop. The few epidemiological studies that have investigated the temporal and spatial dynamics of the disease have been based solely on diagrammatic scales or semi-quantitative indices. To reveal epidemiological processes involved in the development of CCS epidemics, we investigated pathometric relationships. To this end, standardised measurements were defined (disease incidence i, lesion density d, conditional lesion density cd, lesion size ls, and total diseased area tda). The evolution of a cohort of CCS lesions according to their size suggested that lesions can expand over time. Two pathometric relationships were tested: a first one, between i and tda, is given by the equation i=100(1−exp(−a(t)tda)), where t is thermal time, and a second one, between tda, d, and ls, is given by the equation tda=c(t)πd(ls/2) 2 . These relationships were validated for CCS epidemics in the case of field experiments, a survey in commercial fields, and a controlled-conditions experiment. The temporal linear decrease of the time-dependent parameter a(t) in the first relationship suggested that CCS epidemics followed classical epidemiological phases driven by successive processes: (1) the mobilisation of soil inoculum leading to primary infection, (2) the spread of disease to neighbouring taproots (alloinfection), and (3) the intensification of disease on the taproot (autoinfection). This is consistent with complementary experimental results which demonstrated that auto-and alloinfections occur in CCS epidemics.

Incidence and Severity of Cavity Spot of Carrot as Affected by Pigmentation, Temperature, and Rainfall

“…An increase in the size of lesions for most cultivars was observed as the season progressed but, for some orange, purple, and yellow cultivars, the severity of CS was high early in the growing season (midseason assessment) and lower postharvest. These changes are consistent with the findings of Suffert and Guibert (41), Suffert and Montfort,(42,43) and Suffert et al (44), who studied the mechanisms of carrot infection by Pythium spp. and CS field epidemics.…”

Lateral roots of carrot have a low impact on alloinfections in cavity spot epidemic caused by Pythium violae

“…In treatment N+/R+, the mycelium of P. violae could grow through the nylon mesh; however, contact between lateral roots of the donor and of the receptor root systems was greatly reduced, and mycelial growth was potentially enhanced by exudation from the presence of additional roots. CCS lesions were scored using standardised measurements defined by Suffert and Montfort (2008): i is the disease incidence, tda is the total diseased area, d is the lesion density, and si is the symptom intensity. Statistical analyses were carried out using the SAS statistical package, version 8.1 (SAS Institute 2000).…”

“…Three complementary methods were used. The first approach was a deductive one to look at the decrease over time of a time-dependent parameter involved in the relation between incidence and severity (Suffert and Montfort 2008). On the basis of the results, CCS epidemics seemed to be driven successively by (1) the mobilisation of soil inoculum, leading to primary infection; (2) the spread of disease to neighbouring taproots (alloinfection); and (3) the intensification of disease in the taproot (autoinfection).…”