1. Plant Diseases Plant diseases reduce the harvest of food
worldwide by about 30% each year.
Diseases can take many forms, and
cause many different types of symptoms
in host plants.
Ex: necrosis (death), wilts, loss of fruit,
economic damage to harvested crops

2. Tomato early blight (Alternaria solani)
-caused by a soil-borne fungus, can over
winter in plant debris on the ground.
-causes dark sunken lesions on mature
fruit, concentric rings of necrosis, fruit
often falls from the plant before
harvest.
-also causes defoliation of the host plant

3. Tomato Early Blight,
Altenaria solani

5. 2) Grey Mold (Botrytis cinerea)
caused by a soil-borne fungus, easily
invades new tissue on many plants.
-requires high humidity, close proximity
of other plants.
-attacks stems first, causes black slimy
lesions, which cause the plant to die.
-lesions on fruit cause economic loss.

6. Grey Mold of bean, (Botrytis cinerea)

7. Botrytis mold on raspberries
and strawberries

8. 3) Corn Smut (Ustilago zeae)
-overwinters in soil as spores, which
can survive in soil for over 3 years.
-causes ears of corn to turn black, with
a greasy appearance. Large amounts of
spores are produced within the infected
ears.
-these spores can be blown on the wind to other plants.

9. Corn smut, Ustilago zeae

11. -one of the most devastating plant diseases, it is responsible for the
4) Potato Late Blight, (Phytophthora
infestans)
-one of the most devastating plant
diseases, it is responsible for the
Irish Potato Famine of the 1800’s.
-caused by a fungus, it will defoliate an
entire field of potatoes in a few days.
-tubers are also infected, and cause the
disease to be transmitted over years.

12. Potato Late Blight, (Phytophthora infestans)
Defoliated Plant
Infected tuber

14. 5) Dutch Elm Disease, (Ceratocystis
ulmi)
-a devastating fungal disease of elms,
was first discovered in the U.S. in the
1930’s.
-a vascular wilt pathogen, it cuts off the supply of water and nutrients to the affected limbs.
-vectored by the Elm Bark Beetle
-has caused a virtual elimination of Elms

15. A healthy Elm
A diseased Elm

16. The Elm Bark Beetle
A Bark Beetle Feeding
On an Elm branch.

17. A disease resistant
Chinese Elm

18. 6) Black Spot on Rose (Diplocarpon
rosae)
-a fungal disease, it causes black spots
to form on leaves. The spots eventually
cover the entire leaf, which then falls
off. All of the leaves will fall eventually.
-defoliation of the plant leaves it open
to drought and insect damage.
-damaged plants may not live next year.

19. Black Spot on Rose (Diplocarpon rosae)

21. 7) Fire Blight of Apple and Peach
(Erwinia amylovora)
-a bacterial disease, spread by rain, as
well as by insect vector.
-infects through the flowers, in early
spring.
-causes a distinctive “shepherds crook”
on infected limbs. Limbs appear burned
and the entire tree will eventually die.

22. Fire Blight of Apple and Peach (Erwinia amylovora)
Apple Limbs
With Fire blight
Peach Limbs
With Fire blight

25. Control of Plant Diseases
There are a variety of methods for the
control of plant disease. They fall into
a few broad categories.
Cultural control: the use of specific
techniques or practices to eliminate
or mitigate the pathogen.
Ex:disinfecting pruning equipment, removal
of dead plants and plant debris, spacing of
crops to allow drying, crop rotation

26. 2) Biological Control: using plants
that are immune to the pathogen, or
are at least tolerant.
Ex: the planting of Chinese Elms
at the UD Campus, instead of
American Elms.
-Using biopesticides (beneficial bacteria)
which outcompete some diseases (mostly
Erwinia)

27. 3) Chemical control: the use of some
pesticide to kill either the pathogen
or its vector. This usually has to be
done over the course of a season, not
just once.
Ex: Spraying fields to kill off the late blight
fungus, spraying antibacterial chemicals on
fire blighted trees, injecting American Elm
trees with insecticide and fungicide to kill
the fungus AND the vector.