Miscellaneous Pests - Pests of Ornamental Trees, Shrubs and Flowers - page 430

NEMATODES

Leaf nematodes ( 1146 )
Leaf nematodes ( Aphelenchoides spp.) affect a wide
variety of hosts, occurring as ectoparasites or
endoparasites of the buds, leaves and growing points;
they sometimes also feed in the epidermal layers of
green stems. The pests often attack greenhouse plants
and propagation material, and they thrive in warm,
moist conditions. Chrysanthemum nematode
( Aphelenchoides ritzemabosi ) is a major pest of
Chrysanthemum and one of the most important species
to attack ornamentals. Adults are about 1 mm long, and
usually overwinter in the dormant buds and shoot tips of
chrysanthemum stools. They are capable of surviving in
dried leaves for several years, but survival in the soil is
poor. The nematodes slither over host plants in films of
water and usually enter their hosts through stomata.
They reproduce sexually, and development from egg to
adult takes about two weeks. Affected tissue becomes
yellowish and then brown or blackish, these symptoms
often appearing as distinctive wedge-shaped areas
between the major veins. Seriously affected leaves
eventually wither and die. In moist conditions
symptoms spread rapidly up the plant, and new leaves
emerging from infested shoots are often distorted
and thickened; heavy infestations lead to stunting of
growth and the production of small, malformed
flowers. Other plants attacked by this species include
African violet ( Saintpaulia hybrida ), buddleia
( Buddleja ), Ceratostigma , Cineraria , Crassula , Dahlia ,
Delphinium , Doronicum , lavender ( Lavendula ),
Michaelmas daisy ( Aster ), peony ( Paeonia ), pyrethrum
( Chrysanthemum coccineum ), wallflower ( Cheiranthus
cheiri ) and Weigelia . Various other species of
Aphelenchoides are also pests of ornamentals. These
include: A. blastophthorus , primarily a pest of scabious
( Knautia and Scabiosa ) but also associated with plants
such as Anchusa , Anemone , Begonia , bulbous Iris ,
Cephalaria , globe flower ( Trollius europaeus ), lily-of-
the-valley ( Convallaria majalis ), Narcissus and sweet
violet ( Viola odorata ); A. fragariae , which affects
mainly Liliaceae, Primulaceae, Ranunculaceae and
many kinds of fern; and A. subtenuis on Phlox and
various bulbous hosts, including Allium , Colchicum ,
Crocus , Narcissus , squill ( Scilla ) and tulip ( Tulipa ).

Nematodes are important pests of ornamental plants,
and some species are of significance as virus vectors.
Brief details of the main groups associated with
ornamentals are given below.

Cyst nematodes
Cyst nematodes are associated mainly with the roots of
herbaceous plants. Affected plants are weakened, and
often become stunted and discoloured; they may also
wilt in strong sunlight. The root system is typically
attenuated, plants often developing a mass of new
rootlets ('hunger roots') close to the soil surface in an
attempt to compensate for the reduction or loss of
normal root activity. Cyst nematodes invade the young
roots of host plants as minute second-stage juveniles,
the first-stage having moulted whilst still within the egg.
Having gained entry to the host, each nematode settles
down to feed in the centre of a young root. Feeding
induces the development of giant cells which interrupt
normal vascular activity within the root. Some of the
nematodes develop into small worm-like males which
eventually escape into the soil. Females, however,
mature into whitish or yellowish, lemon-shaped
(typical of the genus Heterodera ) or rounded bodies,
c. 0.5-1.0 mm long, which burst through the root
surface but remain attached at the head end. After
mating, the females die and their bodies darken into
hard-walled protective cysts packed with minute, oval
eggs. These cysts eventually break away from the
roots and drop into the soil. Hatching of the eggs is
often dependent upon the presence in the soil of
chemicals exuded from the young roots of host
plants. Cysts commonly remain viable in the soil for
many years before eventually releasing the infective
second-stage juveniles. Species of Heterodera most
often noted on ornamental plants include: H. cacti on
poinsettia ( Euphorbia pulcherrima ) and various cacti;
H. cruciferae on wallflower ( Cheiranthus cheiri )
and other brassicaceous plants; H. fici on Ficus ,
including rubber plant ( F. elastica ) and weeping fig
( F. benjamina ); and H. trifolii on carnation ( Dianthus
caryophyllus ).