Tuesday, 3 December 2020
Name: Foliar nematode, Aphelenchoides fragariae
Aphelenchoides fragariae is a foliar nematode which is widely distributed through the tropical and temperate zones around the world and is the primary target of quarantine checks for imported strawberry material throughout Asia, Russa, and the USA. It is a frequent and economic pest of strawberries, foliage and nursery plants. Over 250 plants in 47 families have been identified as a host of this nematode, hosts include strawberry, fig, aster, begonia, anthurium, 100 fern varieties, members of Liliaceae, Primulaceae and Ranunculaceae families.
Similar to: Aphelenchoides ritzemabosi that occurs mainly in chrysanthemum, and Aphelenchoides besseyi which causes “crimp disease” in strawberries.
Aphelenchoides fragariae is a bisexual species that is both ectoparasitic (lives outside the host) and endoparasitic (lives inside the host) depending on the host.
A complete lifecycle takes about 10 to 11 days at 18oC and survival is dependent on temperature and available moisture. At sub-zero temperatures it will survive for about 3 months in the soil or longer if inside plant material, and even multiply at temperatures as low as -2oC, at 20oC the population will increase quickly. They move through surface water on leaves and are spread to other plants in water slash.
On strawberries, the nematodes attack above-ground parts of plants feeding on and destroying the mesophyll cells within the leaves resulting in typical leaf-blotch symptoms and reddening along the veins. This can cause the entire leaf blade to appear red, especially on strawberry varieties Royal sovereign, Laxton’s King George, Duke and Aberdeen Standard.
Endoparasitic feeding causes abnormal growth, stunting and deformation of buds, leaves and flowers, which is the first symptom. Shoots and leaves can appear twisted and puckered, discoloured areas become hard and rough, undersized leaves develop crinkled edges, petioles redden, runner internodes shorten, flower trusses are reduced, and crown bud die. Ectoparasitic feeding on folded crown and runner buds causes small dry, brown feeding areas; occasionally the nematodes are found in strawberry fruit pulp.
On flowering plants, leaf feeding areas appear as irregular, water-soaked patches later turning brown, violet or purple. In Lilies they cause die-back disease in which leaves, flower buds and fruits turn brown and die. Decay of buds of tree peonies in Japan, and on Philippine violet (Barleria cristata) symptoms begin as chlorotic vein delineated areas which later change to light brown, then dark brown and finally black.
On ferns, leaf-blotch symptoms appear during winter when vegetative growth is lowest. Western Sword-fern (Polystichum munitum) show typical water-soaked stripes on fronds that turn brown in summer when the fern forest dries out. Leaf blotches or stripes often look chevron-like.
Refer to the nematode factsheet for more information.