PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Tomato big bud (greening)
An example of a phytoplasma disease
Also called greening, rosette or virescence
Cause
Candidatus Phytoplasma aurantifolia.
Note that although phytoplasmas are more closely
related to bacteria, they are dealt with here because
they behave more like viruses.
Host range
Very wide. Species affected include:
Ornamentals, eg aster, chrysanthemum, dahlia,
geranium, larkspur, marigold, petunia, phlox,
snapdragon, shasta daisy, zinnia.
Vegetables, eg tomato, potato, eggplant,
capsicum, lettuce.
Field crops, eg clovers, lucerne, tobacco.
Weeds, eg crowsfoot, dock, lamb's tongue,
nightshade, spear thistle, sowthistle, thornapple.
Symptoms
Different symptoms develop on different hosts.
Phytoplasmas infect plants systemically.
Vegetables.
Tomato Symptoms may not develop for
6 weeks or longer after infection. Stems become
thick and the plant has a stiff upright appearance
(Fig. 146). Plants branch prolifically to produce
many stiff shoots, with shortened internodes,
giving the plant a bushy appearance. Root initials
may develop high on the stem and splitting may
occur. Flower buds are greatly enlarged and
imperfectly developed. The sepals often fail to
separate and the whole bud is green. Abnormal
flowers do not set fruit. Fruit, immature at the
time of infection, becomes distorted with a large
woody core. Fruit production is greatly reduced.
Slightly raised white surface areas may develop in
an irregular pattern.
Potato (purple top wilt). A rolling and
pigmentation of upper leaves and erect leaf
stalks. Leaves of white flowered varieties turn
yellow, leaves of pigmented varieties turn red or
purplish depending on the variety. Leaf
pigmentation intensifies and stems also become
pigmented. Crops grown under high moisture
develop a bunched appearance. Stems eventually
yellow and collapse, the lower stems showing
internal browning. Flowers. There is no
greening. Tubers may be flabby and may show
discoloration at the stem end. Tubers from
infected plants may form spindly shoots.
Ornamentals, other hosts.
Some or all the petals are green instead of their
usual colour, hence the name ‘virescence’ which is
often used (Figs. 146, 147). There may be a
proliferation of shoots, plants look bushy. There is
no bud enlargement. Plants may be stunted.
Diagnostics.
Where tomato big bud is suspected commercial
growers can submit samples to a diagnostic
service for confirmation (page xiv).
The greening symptoms and bushiness may be
mistaken for herbicide injury or genetic causes.
‘Greening’ usually only infects herbaceous plants.
A few species of flowers are naturally green, eg
green rose (Rosa chinensis viridiflora, Bells of
Ireland (Molucella sp.).
Some senescing flowers are greenish, eg
hydrangea, arum lily (Zantedeschia sp.).
Some diseases called ‘greening’ are not
necessarily caused by virus diseases, eg citrus
greening is a bacterial disease.
Other phytoplasmas produce different
symptoms, eg witches broom, aster yellows.
Stiff upright stems. Split stems. Small, distorted green woody fruit.
Fig. 146. Tomato big bud - symptoms on tomato. PhotoCIT, Canberra (P.W.Unger).
Virus and virus-like diseases 289