PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Downy mildews
The most famous downy mildew disease is that
caused by a fungus (Plasmopara viticola) on grape
vines which devastated French vineyards in the
l890s. The trouble started when, to control a gall
aphid, the grape phylloxera (Daktulosphaira
vitifolii); resistant rootstocks were imported from
North America. Downy mildew was apparently
introduced on these, and although this disease was
not destructive in North America, it was on the
varieties grown in France. The disease was not
without some compensation, for it led to the
development of Bordeaux Mixture, arguably one
of the most important fungicides of all time. Check
current registration status in your State/Territory.
Scientific name
Downy mildews (Order Peronosporales, Phylum
Oomycota). Common downy mildews include:
Bremia lactucae
Lettuce
Hyaloperonospora parasitica
(formerly Peronospora parasitica)
Brassicas, eg cabbage,
stock
Peronospora antirrhini Antirrhinum
P. destructor Onion
P. sparsa Rose
P. tabacina Tobacco (blue mould)
Plasmopara viticola
Grape
Pseudoperonospora cubensis Cucurbits
Sclerophthora macrospora Grasses, cereals
See also page 320.
Host range
Ornamentals, eg bedding plants, eg stock, sweet
pea, poppy, ranunculus, roses, etc.
Fruit, eg grape. Vegetables, eg cucurbits,
lettuce. Field crops, eg wheat. Weeds.
Generally a particular species of downy mildew is
restricted to one host, or group of related hosts, eg
one species attacks roses and another cucurbits,
and so on. Strains of some downy mildews exist
and new strains may be continually evolving so
that new resistant varieties and fungicides may be
continually required. Races of Peronospora
parasitica f.sp. matthiola will only infect stock
and not other Brassica spp.
Symptoms
Leaves, stems, petals, flower stalks, buds, fruit and
pods may be attacked. Seedlings may be killed.
Leaves, shoots. Most obvious symptoms appear
on leaves but vary with the host. Leaves may fall.
As lesions dry out during dry weather, they shrivel
and die and entire plants may be killed if attacked
early in the season. Systemic infection may occur
in some hosts, eg antirrhinum, cereals, rose, causing
yellowing of growing tissues, distortion, leaf
russetting and stunting (impatiens).
Upper surface. Pale green to yellow irregular
shaped areas/spots, usually delineated by veins
develop on the leaf upper surface, they may
enlarge, coalesce and cover large areas of the
leaf. Depending on the host, these spots vary from
light green to red to brown dead areas, eg on
roses purple areas are evident while on stock and
pansies, yellow areas are prominent.
Lower surface. Under high humidity a typical
white or gray downy/fluffy fungal growth may
form on the underside of infected tissue. Sometimes
there is insufficient fungal mass to be seen
even with a magnifying glass, eg pansies (spores
mauve), snapdragon, Brassicas (spores white).
Roses develop red-black spots on leaves, petals
and stems in advance of obvious mildew. Purple
areas on leaves turn pale brown. Leaves may fall,
even when other symptoms are not obvious.
Infection of young apical shoots causes
distortion, stunting and stem cracking.
Fruit. Downy mildew spores may develop on
fruit, eg grapes which later shrivel.
Secondary infections. Downy mildews are
less common than powdery mildews, but secondary
infections may follow, eg Botrytis, bacterial slime
and rotting (lettuce).
General. Downy mildew can be common and
destructive in favourable conditions leading to total
crop loss. Some downy mildews are more
aggressive than others, eg downy mildew on
snapdragon seems to spread faster than on pansy.
Nursery seedlings can be seriously affected.
Fig. 187. Downy mildew of lettuce
(Bremia lactucae). White angular patches
of fluffy fungal growth on undersurface
of leaf. Photo NSW Dept of Industry and Investment.
Fig. 188. Downy mildew of grapes (Plasmopara viticola).
Left: Fungal spores on undersurface of leaf. Right: Lesions on
uppersurface of leaf and on fruit. Photo NSW Dept of Industry and
Investment (M.Senior).
348 Fungal diseases - Examples of fungal diseases