Demonstration of secondary infection by Pythium violae in epidemics of carrot cavity spot using root transplantation as a method of soil infestation

Abstract

Cavity spot of carrot (CCS), one of the most important soilborne diseases of this crop worldwide, is characterized by small sunken elliptical lesions on the taproot caused by a complex of pathogens belonging to the genus Pythium , notably P. violae . In most soilborne diseases the soil is the source of inoculum for primary infections, with diseased plants then providing inoculum for secondary infections (both auto-and alloinfection). Using fragments of CCS lesions to infest soil, it was demonstrated that CCS lesions on carrot residues can cause primary infection of healthy roots. Using a novel soil infestation method, in which an artificially infected carrot root (the donor plant) was placed close to healthy roots (receptor plants) the formation of typical CCS lesions were induced more efficiently than the use of classical soil inoculum and showed that CCS can spread from root to root by alloinfection from transplanted diseased roots. The method also demonstrated the polycyclic nature of a CCS epidemic caused by P. violae in controlled conditions. Secondary infections caused symptoms and reduced root weight as early as two weeks after transplantation of the diseased carrot. This reproducible method may be used for delayed inoculation and for studying the effect of cropping factors and the efficacy of treatments against primary and secondary cavity spot infections.

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

“…The present experiment confirmed that a carrot taproot quickly produces new lateral roots and leaves regrow 2 weeks after transplantation (Suffert and Montfort 2007). Concomitantly with CCS lesions on taproots, brown to rustcolored microlesions were sometimes observed on carrot lateral roots, but Pythium species were rarely isolated from them (Suffert and Guibert 2007).…”

“…The production of secondary inoculum of Pythium sp. has already been established (Burdon and Chilvers 1975;Green and Jensen 2000;Suffert and Montfort 2007); however, no study has firmly differentiated between spread by mycelia from that by zoospores. Microscopic examinations of P. ultimum in the bulk potting mix confirmed that the spread was due to active mycelial growth because no zoospores were observed in the soil (Green and Jensen 2000).…”

“…The leaves were cut 1 cm above the soil surface 18 days after. The receptor taproots were harvested 2 weeks later, i.e., 5 weeks after the transplantation of the donor taproots because in prior experiments several alloinfections occurred within 4 weeks after transplantation (Suffert and Montfort 2007).…”

“…Fresh store-bought Nanco carrots were used as donor taproots. They were wounded at two spots (each 1-2 cm 2 ) with abrasive tissue, and a mycelial plug (5 mm in diameter), extracted from a 7-day-old P. violae culture, was placed on each wound (Suffert and Montfort 2007). Plugs were held in place with sterile pins.…”

“…For the third approach, a purely experimental approach, diseased carrot taproots were transplanted close to healthy roots in a greenhouse environment. P. violae spread from taproot to taproot, firmly establishing the occurrence of alloinfection and the polycyclic nature of a CCS epidemic (Suffert and Montfort 2007).…”