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Test is next Thursday, Feb 12
• All the material on the website is on the
exam including the lecture on Feb 5.
• Material covered on Feb 10, will be not be
on the first exam.
Today
• Fungal like organisms
Some of this lecture material was created by U. of Arizona, Purdue and AVRDC
KSU
Purdue
Horseradish
AVRDC
Cabbage
AVRDC
Kingdom Chromista
Oomycetes
Not true fungi – phylogenetically unrelated to
Ascomycetes, Basidiomycetes, Zygomycetes, and
Chytridiomycetes
Characteristics (generalizations)
1. Mycelium is coenocytic (without
crosswalls)
2. Hyphal wall is composed of cellulose
3. Produce oospores (sexual spore)
as a resting stage
Slide courtesy of Barry Pryor, U. Arizona
Characteristics, continued…..
4. Produce zoospores (motile) or
similar non-motile asexual spores
in sporangia
5. Most are diploid (2N) – whereas
most true fungi are haploid
6. Sexual reproduction by
gametangial contact between
oogonia & antheridia
Slide courtesy of Barry Pryor, U. Arizona
Oomycetes cause two types of
general diseases
1. Diseases affecting plant parts in the soil or
in contact with soil
2. Diseases that affect above ground parts –
leaves & stem
Both categories require
the presence or abundance
of water
Slide courtesy of Barry Pryor, U. Arizona
Primary groups of Oomycetes
I. Aphanomyces spp. – root rots
II. Pythium spp. – damping –off; root rots & soft
rots
III. Phytophthora spp. root rots; damping-off,
blights; other rots
IV. Downy Mildews*: foliage blights
V. White Rusts*: foliage diseases
*Obligate parasites
Slide courtesy of Barry Pryor, U. Arizona
Pythium seed rot, damping off,
root rot, and soft rot
Caused by many Pythium spp.
P. ultimum, P. debaryanum, and P. viole
are some common species
Damping off
1. Seeds fail to germinate
2. Before emergence (pre-emergence
damping off)
3. Seedling emerged are usually attacked
from the roots first.
Invaded areas become water soaked and
discolored. The base of the stem becomes
thinner and the seedling falls over.
Slide courtesy of Barry Pryor, U. Arizona
Pythium blight of cereals & grasses
•Empty patches in field or on lawn
•Stunted growth or delayed maturation
•Several species of Pythium cause diseases, but other
organisms can cause similar symptom
Slide courtesy of Barry Pryor, U. Arizona
Plant Pathology, G.N. Agrios
Zoospores Liberated
http://www.apsnet.org/online/Archive/2003/IW000025.asp
Phytophthora Diseases
Diseases on many types of plants and is similar to Pythium.
Species of Phytophthora are important pathogens of woody
ornamentals, field crops, ornamental plants, fruits, etc.
examples:
P. fragariae – red stele of strawberry
P. sojae – soybean root rot
P. infestans – late blight of potato
P. ramorum – sudden oak death
General Symptoms of Diseases
Caused by Phytophthora
* Root or lower stem rots
* Loss of feeder roots
* First symptoms are drought wilt or mineral deficiencies
Generally – high soil moisture + cooler temperatures promote
disease - < 23ºc
Annuals or tree seedlings may be killed in days or weeks.
Older plants exhibit sparse foliage, dieback of branches
or twigs, and/or yellow leaves – sometimes collar rots and
necrosis of lower stem
Slide courtesy of Barry Pryor, U. Arizona
Late blight of potato
Phytophthora infestans
Symptoms – water soaked lesions on lower
leaves brown blighted areas within +
indefinite borders – worse in wet, cool
weather.
Can affect leaves & tubers (purple, stinky
rot)
Control – sanitation, resistant varieties,
timed chemical sprays
Slide courtesy of Barry Pryor, U. Arizona
Late blight of potato: Phytophthora infestans
Disease cycle
Downy Mildews
- obligate biotrophic parasites
- foliar blights
- produce sporangiophores + sporangia in very
characteristic ways for each genus of downy mildew.
examples:
Pseudoperonospora cubensis – downy mildew of cucurbits
Plasmopara vitacola – downy mildew of grape
Slide courtesy of Barry Pryor, U. Arizona
Haustoria of P. viticola
Slide courtesy of Barry Pryor, U. Arizona
Downy mildew of grape:
Plasmopora viticola
Symptoms are small, chlorotic spot on both sides
of leaves.
Signs are sporangiophores that emerge through the
stomates. The underside of the leaf turns white.
Spots later may coalesce and form large necrotic
areas in severe infections, plants may be defoliated.
The pathogen can also attack the fruit and the
budwood. The budwood will allow the pathogen to
overwinter in the vineyard.
Slide courtesy of Barry Pryor, U. Arizona
Sporangiophore emerging from stomata
Slide courtesy of Barry Pryor, U. Arizona