In the nineteenth century, farmers had already learned that the yield of sugar beet decreased steadily when beet crops were grown repeatedly in narrow rotations. Schacht showed in 1859 that this damage was caused by beet cyst nematodes, Heterodera schachtii, a parasite which has been a limiting factor in beet production until the present day. There are other nematodes that cause considerable damage in sugar beet but H. schachtii is the most important one. The insects Atomaria linearis (pygmy mangold beetle), Agriotes obscures (wire worm), Tipula paludosa (leatherjacket), Chaetocnema tibialis (fleabeetle) and Onychiurus armatus (springtail) cause seedling death. Sucking damage to the seedling leaves is caused by Calocoris norvegicus (potato bug) and Piesma quadrata (rape leaf bug). Pegomyia betae (beet fly) is a leaf mining insect.
Important viruses are Beet necrotic yellow vein virus, causing the disease Rhizomania, which is vectored by the fungus Polymyxa betae, and Beet mild yellows virus which is vectored by the aphid Myzus persicae. Major soil-borne fungal diseases in sugar beet include Rhizoctonia solani, Aphanomyces cochlioides, Fusarium spp., and Verticillium dahliae. Above-ground, sugar beet leaves are attacked by Erysiphe betae (mildew), Uromyces betae (rust), and the leaf spotting pathogens Ramularia beticola and Cercospora beticola.
Damage by and prevalence of Cercospora beticola>
In severe epidemics, CLS causes progressive destruction of leaves, followed by a continual replacement of new leaves at the expense of stored reserves in the root, leading to a reduction in yield and sugar content. In the Netherlands, yield reductions of up to 21% in sugar yield (sugar content x root yield) and 23% in financial return were observed (this thesis), but for individual beet fields, sugar yield reductions of up to 40% have been observed. This results not only in less income for farmers, but also for the sugar industry as extractability of sugar is less with infection by CLS. The percentage of beet fields in the Netherlands in which a CLS infestation is observed has increased from 13% in 2000 up to 32% in 2002. Consequently, the percentage of beet fields in which a fungicide spray against C. beticola was applied increased from 9% in 2000 up to 23% in 2002.
The fungus Cercospora beticola
Cercospora beticola Sacc., causal agent of Cercospora leaf spot (CLS) in sugar beet is the most common and destructive foliar disease of sugar beet worldwide . The fungus belongs to the class of Fungi Imperfecti (Deuteromycetes), order Moniliales, family Dematiaceae, section Phaeophragmosporae. Hyphae are hyaline to pale olivaceous brown, intercellular, septate, 2-4 μm in diameter, and form pseudostromata in substomatal cavities of the host from which conidiophores are borne in clusters. Conidiophores, emerging only from host stomata, are 10-100 (mostly 46-60) μm x 3-5.5 μm and unbranched, with small conspicuous conidial scars at the geniculations and the apex. Conidia, 20-200 x 2.5-4 μm (mostly 36-107 x 2-3 μm), are smooth-walled, straight to slightly curved, hyaline, acicular, and gradually attenuated from the truncate base, with 3-14 (sometimes up to 24) septa. No teleomorph stage of C. beticola is known at this moment.
Epidemiology and life cycle of Cercospora leaf spot on sugar beet
The symptoms of Cercospora leaf spot (CLS) consist of delimited, circular spots which develop on older leaves, and enlarge to 2-5 mm when mature. Lesions are tan to light brown dark with brown or reddish-purple margins. Elongated lesions occur on petioles, and circular lesions may occur on sugar-beet hypocotyls not covered by soil. Individual spots on the leaves coalesce as the disease progresses, and large areas or leaves become brown and necrotic. Minute black dots, pseudostromata, are often visible in the centre of mature lesions. Under humid conditions, conidiophores are formed on the pseudostromata, and the leaf spots become grey and velvety with the production of conidia (Figure 1.7). Blighted leaves eventually collapse and fall to the ground but remain attached to the crown. The younger heart leaves are usually diseased later than older leaves. All above-ground parts of seed plants including seed clusters, can be affected.
Cercospora beticola is inactive at temperatures below 10°C, and can infect between 12-37°C. Optimal temperatures for conidia production are between 20-26°C if the relative humidity is in the range of 98-100% or at 25°C . Severe epidemics can be expected when the relative humidity stays above 96% for 10-12 hours each day for 3-5 days and the temperature remains above 10°C. Despite these rather high temperatures, severe epidemics of Cercospora leaf spot can develop in the Netherlands. Conidial release is effected by rain and dew (Meredith, 1967) and conidia are mainly disseminated by rain-splash, and less efficiently by wind, irrigation water, insects and mites.
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