PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
EXAMPLES OF FUNGAL DISEASES
Powdery mildews
Powdery mildews are considered to cause more
financial losses worldwide than any other plant
disease. Historically the most famous powdery
mildew disease is the one that devastated the vine
crops in Europe during the l9th century and is still
costly to the wine industry. It was the same fungus
which led to the discovery of lime sulphur - sheer
necessity! This fungicide is still used today for
powdery mildews and to a lesser extent rusts, other
fungal diseases and mites.
Scientific name
Powdery mildews (Order Erysiphales, Phylum
Ascomycota). However, when the sexual stages
(cleistothecia) of powdery mildews are not known,
they are placed in the Imperfect Fungi and called
Oidium spp. Common powdery mildews include:
Phylum Ascomycota, Order Erysiphales
Blumeria graminis Cereals, grasses
Podosphaeria leucotricha Apple
Sphaerotheca fuligina Cucurbits
S. pannosa Rose
Uncinula necator Grapevines
Imperfect Fungi (sexual stage not known,
when found the fungus is given a genus and species).
Oidium spp. – Aster, azalea, begonia, calendula,
chrysanthemum, dahlia, euonymus, eucalypt, oak,
pansy, plane tree, primula, other plants.
See also page 321.
Host range
Ornamentals, azalea, begonia, hebe, eucalypt,
wisteria, hardenbergia, oak, rose.
Fruit, eg apple, grape, papaya, strawberry.
Vegetables, eg cucurbits, pea, tomato.
Field crops, eg cereals, clover, lupins.
Turf, eg grasses, clover.
Parasitic plants, eg mistletoe. Weeds.
Although all powdery mildews look the same,
usually a particular species is restricted to one host,
or group of related hosts, eg one species attacks
roses another azaleas and so on.
Fig. 185. Powdery mildew of euonymus
(Oidium spp.). PhotoCIT, Canberra (P.W.Unger).
Symptoms
Leaves, stems, buds, petals. The first sign
of disease is usually small white circular patches on
the surface of leaves or stems. These increase in
size, often running together to cover large areas of
both upper and lower leaf surfaces, becoming
powdery due to the production of masses of conidia.
Young leaves on some species seem to be
very susceptible and may yellow, shrivel and
curl, eventually they may die, eg apple.
However, in most cases, younger leaves of
bedding plants do not show infection.
Petals and buds may also become distorted.
Flowers are downgraded.
Infected leaves on some hosts redden in
colour on the uppersurface opposite a powdery
mildew colony on the undersurface and may
be confused with chemical toxicity. Leaves may
wither and fall. Infected soft leaves of some
hosts, eg roses, may “bubble” with spores
developing on the deformed areas.
Old powdery mildew infections on some
leaves, eg Photinia, hebe, may appear grayish.
Dormant shoots of apple are covered with
dense white mycelium. Infected shoots on
perennials may die back. Dormant rose and
grapevine shoots may turn reddish so it is easy to
see where the infected shoots from last season are.
Small fruiting bodies (cleistothecia) may
develop on plant tissues killed by powdery
mildew. They look like small black specks.
Fruit. Mango fruit develops purplish brown
blotches and immature fruit may fall. Apples may
russet and be downgraded. Grape bunches with
as little as 5% disease may be rejected by wineries
as they cause ‘off flavor’ in wine. Table grapes are
unmarketable if berries or stalks are infected.
General. Important seedling disease in nurseries.
Plants may be stunted and crops lost. Can be a late
season, end-of-crop disease.
Diagnostics.
Fresh powdery mildews are generally easy to
identify, exceptions include hebe, hydrangea.
Do not confuse with down mildews (page 348).
A few hosts may become infected with both
powdery mildew and downy mildew, eg rose,
grape, hebe, cucurbits.
Microscopic examination - a x10 eyepiece
and x10 objective (student compound microscope)
is needed to see spores in ‘chains’ (page 346,
Fig.186) rather than ‘trees’ (page 349, Fig.189).
Expert advice may be needed to confirm the exact
species although very few plants host more than
one species of powdery mildew.
Active infections appear powdery and fluffy,
while inactive infections appear flattened and
may be brownish.
The purplish discolorations of some powdery
mildews may be confused with chemical toxicity.
On some hosts, eg cucurbits, spots may appear
first on leaf undersurfaces but later cover both
surfaces and growers may not be aware of disease
until it is well established and difficult to control.
Fungal diseases - Examples of fungal diseases 345