Plant Galls

Clubroot

Clubroot is a disease of brassicas (cabbage and mustard family) caused by an amoeba, Plasmodiophora brassicae, which was once classed as a slime mould. It forms multinucleate plasmodia inside infected parenchyma cells of the infected root, and results in the formation of root galls (hence the name).

clubroot 1

Above: an infected cell in an early stage, containing a plasmodium.

clubroot 2

Above and below: plasmodia inside infected cells beginning to divide into a mass of resting spores.

clubroot 3

The resting spores are visible as dark-walled spheres. When the root dies and disintegrates, the spores will be liberated and in favourable conditions they will germinate into zoospores (biflagellate amoebae).

Animal parasites of plants

Fungi and protofungi are the most serious diseases of plants. Many animals parasitise plants,
such as foliage eating insects, and while these may completely destroy herbaceous plants,
they rarely kill trees. Many trees can survive being totally defoliated in bad years.  Oak trees
support a wide range of animal parasites and grazers without any major adverse effect. One
curious example is the acorn weevil,
Curculio, an extraordinary and delightful looking insect
which uses its long curved snout to bore a hole in an acorn, into which it lays its eggs, the
grubs eating the acorn when they hatch. Acorn weevils are one example of the larger family
of nut weevils.

The gypsy moth,
Lymantia dispar, can totally defoliate trees, but oaks and elms are
'designed' to withstand defoliation, for example the oak has lammas shoots, new growth that
appears around mid-sumer and thus replaces any lost foliage. Defoliation occurs in a few
years only and causes decreased tree growth, but generally does not kill trees unless the
trees are weakened by additional stressors like several years of severe drought.

Nematode worms occasionally cause very serious plant diseases, such as the nematode
disease of Japanese pines, which is killing red pines in Japan.

Plant galls. Many insect and mite parasites stimulate the tumour-like growth of plant tissues
in order to nourish and sometimes protect the developing larvae. One example is oak apple
gall. These galls are fairly large (2-5 cm diameter) round apple-like structures that develop
when a gall wasp lays a single egg in a developing leaf bud. The developing larva secretes
plant-hormone mimicking substances which stimulates development of the gall tissue which
nourishes, houses and protects the wasp larva. Similar are the marble galls, hard round balls
about the size of a marble, which also develop when a wasp lays an egg in a leaf bud. The
range of plant galls is astonishing, many occur on fruit, leaf and bud. The complex life-cycles
of gall wasps is also extraordinary and maybe covered in a separate article in future.

Bacterial diseases of plants

One well-known bacterial infection of trees is the crown gall disease, caused by infection by
the bacterium
Agrobacterium tumefaciens, which genetically reprograms neighbouring plant
cells to produce large masses of nourishing tissue. These galls are often visible as large
bulbous outgrowths on tree trunks. The parasitical process is remarkable and may also be
the subject of a future article.

Plant parasites of plants

A number of plants are parasitic or semi-parasitic on other plants. These are covered in a
separate article:
parasitic plants.

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