September/October 2023 - 14

SOIL & PLANT HEALTH
Biotic vs. abiotic
Determining the causes of plant damage
The fungal spores of Botrytis cinera (left), the pathogen causing the gray mold, are a sign of biotic disease. Hail
damage on a watermelon (right), a day after a storm. Photos by Gerald Holmes, Cal Poly Strawberry Center,
courtesy of Bugwood.org.
BY ANN HAZELRIGG
EXTENSION ASSOCIATE PROFESSOR,
UNIVERSITY OF VERMONT
W
hen growers think of plant diseases, they
usually consider those that are caused by living
organisms like fungi, bacteria, viruses and
nematodes. These diseases are termed " biotic " and are
infectious, meaning that under the right environmental
conditions, they can spread from plant to plant.
Many plant disorders can also be a result of " abiotic, "
or non-living issues, such as cold injury, nutritional
deficiencies, drought, mechanical injuries and herbicide
drift or other chemical damage. It can be challenging to
tell the difference between the two kinds of diseases just
by looking at the symptoms. Plant disease symptoms,
whether abiotic or biotic, are typically not specific. For
example, dieback or wilt in the top of the tomato plant
could be a result of a root rot caused by fungi (biotic), a
stem canker caused by a bacterial disease (biotic), or it
could be drought stress (abiotic) - we just need to dig a
little deeper to figure out the cause.
Signs of biotic disease
The first thing I do when trying to diagnose a
problem is to look for signs or physical evidence of
14 | SEPTEMBER/OCTOBER 2023
the presence of infectious pathogens. Signs of fungi
include spores that can be seen with the naked eye
(e.g., powdery and downy mildews), fruiting bodies
(usually with the aid of a 15X hand lens), threads of
the fungus called mycelia or long-term overwintering
structures called sclerotia, as is common in white
mold (Sclerotinia spp.).
Bacterial signs are not as obvious, although you
may see ooze or strands of bacteria as in the case of a
cut cucurbit stem with bacterial wilt. Sometimes you
can only see the signs under a microscope by looking
for bacteria streaming out of the cells.
Viruses leave no signs; we can only guess by
symptoms, which include distorted tissue, mottling,
ring spotting and stunting, among others. For
positive identification, further testing must be done
in laboratories that have the proper equipment.
Nematodes are identified by microscopic examination
and, in some cases, by the presence of cysts or galls as
in root knot nematodes or the soybean cyst nematode.
Signs of abiotic disease
If there are no signs of pathogens present on the plant,
the next step is to consider whether the symptoms are
caused by abiotic or non-living issues. There are several
clues that help identify a problem as abiotic.

http://www.Bugwood.org

September/October 2023

Table of Contents for the Digital Edition of September/October 2023

Editor's Letter
Grower Profile: Managing risk
GLVPN Veg Connection: Powdery mildew
Biologicals: Set to explode
Soil & Plant Health: Biotic vs. abiotic
Food Safety: Food safety check
Great Lakes EXPO News: Emerging technology
Farm Market & Agritourism: Can you support another employee?
Ag Labor Review: The backbone of America
Classifieds
Advertiser Index
September/October 2023 - 1
September/October 2023 - 2
September/October 2023 - 3
September/October 2023 - Editor's Letter
September/October 2023 - 5
September/October 2023 - Grower Profile: Managing risk
September/October 2023 - 7
September/October 2023 - GLVPN Veg Connection: Powdery mildew
September/October 2023 - 9
September/October 2023 - Biologicals: Set to explode
September/October 2023 - 11
September/October 2023 - 12
September/October 2023 - 13
September/October 2023 - Soil & Plant Health: Biotic vs. abiotic
September/October 2023 - 15
September/October 2023 - 16
September/October 2023 - 17
September/October 2023 - Food Safety: Food safety check
September/October 2023 - 19
September/October 2023 - Great Lakes EXPO News: Emerging technology
September/October 2023 - 21
September/October 2023 - Farm Market & Agritourism: Can you support another employee?
September/October 2023 - 23
September/October 2023 - Ag Labor Review: The backbone of America
September/October 2023 - 25
September/October 2023 - Classifieds
September/October 2023 - Advertiser Index
September/October 2023 - 28
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