The Anatomy and Biology of the parasitic Aphelenchi

Abstract

There are three species of true parasites in the genus Aphelenchus, viz.: A. fragariae R.B., 1891, A. olesistus R.B., 1893, and A. phyllophagus n.sp. A. ormerodis R.B., 1891, is a doubtful species, possibly a young form of A. fragariae (Marcinowski, 1908).A. fragariae causes the disease strawberry bunch; A. olesistus and phyllophagus cause leaf disease in flowering plants and ferns. The diseases are widely distributed and of considerable economic importance.From the study of the anatomy of the three species certain points of general importance emerge: (1) The salivary glands of Aphelenchus represent the oesophageal glands of other nematode genera, which have separated from the body of the oesophagus. This separation causes the reduction in size of the posterior portion of the oesophagus, and its lack of clear demarcation from the intestine. (2) The excretory organ of Aphelenchus, and of some other nematode genera, originates from the left lateral line. (3) The vaginae are ectodermal organs distinct in origin from the gonads proper.The life cycle of the parasitic Aphelenchi is divided into three definite stages: (A) That of residence in the definitive habitat, which for A. fragariae is in the stem and leaves of the strawberry plant, for A. olesistus and phyllophagus in the mesophyll spaces of the leaves of many plants. Nutrition and reproduction are most actively carried on in this situation, several or many generations succeeding each other, and here the pathological effects are manifest. (B) The resting stage in the soil. The worms reach the soil in fallen leaves, and the adults survive in a partially dried, dormant condition on the surface of the soil for prolonged periods. (C) The stage of immigration into the host plant. When revived by moisture the Aphelenchi may be attracted by a suitable plant, and wanders on to it; they may live as ectoparasites in the leaf axils, breeding in this situation, the larvae migrating to the definitive habitat; or they may enter the definitive habitat direct. They do not traverse the tissues of the stem during immigration. A. olesistus and phyllophagus enter the mesophyll spaces through the stomata.It should be noted that this life cycle is that of the race, not of the individual. It comprises many generations, and in this respect resembles that of Tylenchus dipsaci and differs from that of T. tritici, which includes one generation only.In the life of the individual Aphelenchus, embryonic development occupies not more than five days, embryonic and larval development not more than ten, and a complete generation not more than fourteen days.High atmospheric humidity assists the migration of the worms, previous disease or injury of the host plant may do so. Some races of various species of plants are immune.Treatment of the affected plants by chemical methods is of limited applicability. Treatment by immersion in water at 50–52° C. for five minutes is recommended by Marcinowski (1908). Prophylaxis should be based on burning of infected plants and leaves, and on the avoidance of infected surface soil in the filling of pots and boxes.