PATHOLOGY OF CYST‐NEMATODES

Summary 1. The cyst‐nematodes, which belong to the genus Heterodera Schmidt, are obligate plant parasites. Several species cause crop diseases of economic importance. These nematodes differ greatly in structure from the other, vermiform, plant‐parasitic and soil‐inhabiting nematodes, for although the migrant larval phase of cyst‐nematodes is vermiform, this phase is only transient. The principal phases are the sedentary female, which feeds upon the plant root, and the cyst, which derives from the female and consists of the eggs surrounded by mucilage within the dead female cuticle. The term ‘egg’ includes the contents within the shell, which at maturity is usually a coiled, second stage, larva. 2. The migrant larva may be parasitized by many species of predacious and endozoic fungi which have been much studied, but these do not parasitize the females or cysts. Until 1974 only one parasite of the females was known, and that imperfectly. On the other hand, many fungi have been cited as parasites of eggs inside cysts, notably the species now known as Phialophora malorum and Cylindrocarpon destructans. Insect‐pathogenic fungi have also been cited as egg parasites but evidence is now against their pathogenicity to Heterodera. No other classes of organism are known to parasitize the females or cysts, although Collembola have been reported as predators. 3. Studies published since 1973, including the present, show a total of seven (or eight) pathogenic fungi. There are two (or three) pathogens of the females, two major and three minor pathogens of the eggs. The former comprise Catenaria auxiliaris and two Entomophthora‐like fungi, possibly the same species but both imperfectly known. The major egg pathogens are Verticillium chlamydosporium and an incompletely known species termed the ‘contortion fungus’ from the symptoms on infected larvae newly hatched from eggs. The minor egg pathogens are a ‘black yeast’ taxonomically fairly similar to but not identical with Phialophora malorum; a sterile ‘crystal‐forming fungus’ and Cylindrocarpon destructans. The main features of these pathogens are described and illustrated. 4. Mycorrhizal fungi belonging to Glomus Tulasne & Tulasne are sometimes associated with cysts. They ordinarily sporulate inside empty cysts, but in one remarkable example from Heterodera schachtii growing on tomato host in glasshouse culture a Glomus apparently behaved as a pathogen. Pythium species may invade cysts maintained under unfavourable conditions. Miscellaneous other fungi are also found occasionally in eggs. 5. In addition to undergoing disease caused by fungi, eggs sometimes undergo oily degeneration. This disorder is not of numerical significance, but a high proportion of cysts are diseased through apparently non‐specific causes whose symptoms can be summarized as ‘lysed, shrivelled, coagulated or decayed’. Although the majority of diseased females are killed by fungal pathogens, some 25–40% of diseased cysts are destroyed through non‐specific causes. Overall disease in ten populations of Heterodera schachtii examined from roots was 9% of females and 26% of young cysts; in 76 populations sieved from soils a mean of 14% of older cysts were diseased. Where the cyst‐nematode host had been grown in monoculture for 11 years higher proportions of cysts were diseased although few pathogens of females were present. Heterodera avenue cysts were parasitized to a similar extent. 6. Of the pathogens of females, Catenaria auxiliaris was widespread in H. schachtii and uncommon in H. avenue, whereas Entomophthora‐like fungi were widespread in H. avenue and rare in H. schachtii. Of egg pathogens, Verticillium chlamydosporium and the ‘contortion fungus’ were common in H. schachtii and H. avenae, and the ‘contortion fungus’ was also found in H. glycines. The minor egg pathogens were much less common.