OBSERVATIONS ON THE BEET EELWORM AND OTHER CYST‐FORMING SPECIES OF HETERODERA

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Abstract
An account is given of observations made between 1944 and 1948 on beet eelworm and related species of Heterodera. The host ranges of beet‐, potato‐, pea‐, carrot‐ and Galeopsis‐root eelworms were studied in the field, on sites where previous information had been obtained of the species of Heterodera present. The beet eelworm attacked plants in the Polygonaceae, Chenopodiaceae, Amarantaceae, Aizoaceae, Caryo‐phyllaceae, Cruciferae, Onagraceae and Labiatae. Considerable differences were observed in the intensity of attack upon different hosts. The Cruciferae contains many efficient hosts and is the most important host family in the British flora. The Galeopsis eelworm, like the beet eelworm, attacked plants in several families, including the Chenopodiaceae, Caryophyllaceae and Labiatae, which also contain hosts of the beet eelworm. The other root eelworms were more specific, the carrot eelworm attacked only cultivated and wild carrots (Daucus carota L.), the potato eelworm attacked only plants in the genera Solanum and Ly copersicum (potato, Solatium dulcamara L. and tomato), while the pea eelworm attacked only peas (Pisutn), beans and tares (Vicia). The potato eelworm did not attack Solanum nigrum L. or S. sarrachoides Sendt. and there was no evidence that potato eelworm is capable of attacking weeds in natural orders other than the Solanaceae. In pot‐tests, the hop eelworm attacked only hops and nettles and the cabbage eelworm only plants in the family Cruciferae. The rates of development of cysts of the beet, potato, pea and carrot eelworms were studied in the field. In all four species, the rate of development was similar and varied with the soil temperature, being slow in spring, more rapid in summer, falling off during the autumn and becoming negligible during the winter. In the pea eelworm, drought caused delay in the invasion of rootlets by eelworm larvae and reduced the intensity of attack. Assuming adequate moisture, the maximum number of generations for all four species is between two and three per year. In practice, the number of generations is also governed by the vegetative period of the host crop. During the host‐range trials and observations on rates of development, fresh cysts were examined in large numbers. Some differences in shape were observed between the larger cysts of the various species. There were also differences in colour and in the number of eggs extruded into the gelatinous egg sac. Analysis of length and breadth measurements suggested that size ranges and regression coefficients are somewhat more valuable than average lengths and breadths, and length‐breadth ratios, which are usually given. The carrot‐root eelworm (Heterodera carotae Jones) is described, and the status of the Galeopsis‐root eelworm is discussed. The latter is distinct from beet eelworm and appears to be identical with a form found by Goffart in 1936, for which the name Heterodera galeopsidis is proposed. The observations here recorded do not support the theory of adaptability of root eelworms to new hosts.