Grower 101: Diagnosing Plant Diseases of Floricultural Crops

November 7, 2002 - 11:53

Need help identifying some common greenhouse diseases? Here are the most common problems.

An accurate diagnosis of disease is important for several reasons.
Bacterial diseases are not controlled with fungicides, and some bacterial
diseases are easily mistaken for fungal diseases. Some fungicides have a narrow
spectrum of activity. In addition, if you know the disease, you can usually
find information regarding the environmental conditions necessary for disease
development. You could also find out if the pathogen was seed-borne or
soil-borne and whether other crops in the greenhouse may also be susceptible.

The ability to make an accurate diagnosis on-site is
dependent on a disease that has unique symptoms. Also, the grower needs to have
previously identified the problem or have a good illustration or written
description to make proper identification. There are a number of diseases that
can be easily identified on-site, and there are many that can only be diagnosed
in a university or private diagnostic lab.

Damping-off and Root Rots. style='font-weight:normal;font-style:normal'> Of all of the diseases,
damping-off and root rot are the most difficult to identify on-site, most often
because wilting, stunting or nutrient deficiency in the leaves distract growers
from the root system that is causing the problems. Pythium and Rhizoctonia are
the two most common causes, but they usually do not leave any clues as to their
identity. Also, they have a wide host range, so the plant on which they occur
does not supply a clue. When Rhizoctonia is causing web blight, you can see the
brown strands of mycelium on the diseased plants. Webbing occurs when
temperature and humidity are high. There are test kits available that can allow
you to make an accurate on-site identification of Pythium, Phytophthora and
Rhizoctonia. Environmental controls: Destroy entire plant and its soil; improve
media drainage; use new containers; plant at appropriate depth.

Soft-Rots. Cuttings have a large wound on the base and can be very
susceptible to Erwinia soft rot, especially when temperatures are in the 90s
and there is plenty of water. When Erwinia is the cause, the base of the
cuttings become soft and slimy. If the cuttings are diseased but remain firm,
something else is the cause.

Poinsettias are subject to a rapidly developing soft rot by
the fungi Rhizopus choenephora and the bacterium Erwinia. These can be
difficult or impossible to identify on-site, but there are no fungicides or
bactericides that will help. Simply discard the plants.

Bacterial Blight of Geranium. style='font-weight:normal;font-style:normal'> Bacterial blight of geranium is
caused by Xanthomonas. Wilted plants will not have root rot or stem canker
(unless there are several diseases occurring). At first, one or two leaves will
become soft and droop. Root rot or high soluble salts can also cause wilt but
usually the whole plant wilts. Another good indication of Xanthomonas is leaf
spots, but leaf spots do not always occur. Ralstonia (Pseudomonas) solanacearum
can also cause a systemic wilt of geranium but does not cause leaf spots. We
rarely see Ralstonia in geranium in New England, but we had several cases in
1999. Xanthomonas only infects geranium but Ralstonia infects a variety of
ornamentals and vegetables (as well as tobacco and banana).

Bacterial Leaf Spots. Bacterial leaf spots can look very similar to fungal
leaf spots but when you hold them up to the light (as if to see the light
through the leaf), bacterial spots usually have a translucent look to them. On
geraniums, you may see a halo around the spot. Bacterial spots on impatiens can
be distinguished from Alternaria leaf spot because Alternaria causes the leaf
to become yellow. Pseudomonas syringae and P. cichorii cause leaf spots on
geranium. Spotted leaves will look similar to those caused by Xanthomonas, but
the leaves will not wilt. Geranium leaves will turn yellow and dry up after
being infected but will not wilt. One way to distinguish bacterial leaf spots
from fungal leaf spots is to consider the environment. Bacteria usually
flourish under hot temperatures; fungi prefer cooler conditions. Environmental
controls: Remove damaged leaves when dry and destroy leaves; keep plants as dry
as possible; use drip irrigation; provide adequate spacing.

Fungal Leaf Spots and Blights. style='font-weight:normal;font-style:normal'> There are many fungi that cause
spotting of floricultural crops. With experience, some can be recognized but
many cannot be identified on-site. Alternaria on zinnia, impatiens and gomphrena
is relatively easy to identify. Botrytis is easy to identify when it is
producing its crop of gray spores. On fuchsia, Botrytis causes cankers, which
usually do not develop spores, but cankers on fuchsia are almost always caused
by Botrytis. Environmental Controls: Remove damaged leaves when plants are not
wet and destroy leaves; use drip irrigation; space for good air circulation.

Powdery Mildew. This disease is easy to identify because of the powdery
crop of spores it produces on the leaf surface, which makes most any infected
plant unsaleable. In most cases, powdery mildew develops on the top side of the
leaf and has a similar appearance on host plants, but on poinsettia and a few
other plants, mildew will also grow on the bottom. Powdery mildew rarely kills
plants but can result in lower leaf drop.
Occasionally, plants will develop purplish discolorations as a result of
severe infestation. African violet, begonia, dahlia, gerbera, hydrangea,
kalanchoe and pansy commonly develop powdery mildew. On kalanchoe, powdery
mildew can be difficult to recognize because only a fine webbing will develop.
Environmental controls: Use resistant varieties; avoid crowding plants; grow
plants in full sun if possible; keep plants as dry as possible.

Rusts. Like powdery mildew, rust diseases are easy to identify.
The rust fungi produce pustules of spores on the bottom side of the leaf and
pale spots on the upper Á leaf surface. The pustules contain masses of
rusty to orange-colored spores. Snapdragon, fuchsia, carnation and geranium are
among the plants susceptible to rust diseases. To diagnose rust diseases, rub
infected leaves on a sheet of white paper; the rust-colored streaks left behind
by spores are diagnostic for rust. Environmental controls: Remove damaged leaves
when plants are dry and discard leaves; keep plants dry; avoid crowding plants.

Fusarium and Verticillium Wilt. style='font-weight:normal;font-style:normal'> Diseases caused by these fungi
usually affect only part of the plant at first. Foliage on the affected stems
will wilt and usually become yellow. There may or may not be discoloration in
the vascular tissue (cutting into the stem will reveal a dark stain). It is
difficult to diagnose these diseases with much confidence in the field. With
cyclamen, several leaves will turn yellow. When you cut through the corm (cut
the top off) you will see a dark band of vascular tissue that corresponds with
where the yellowed leaves were. These two vascular wilt fungi are not commonly
seen in greenhouses. When they do occur, it is usually on chrysanthemum, cyclamen
or basil. Environmental controls: Destroy entire plant and soil; improve soil
drainage; use new soil.

Viruses. Virus diseases can be either very distinctive or impossible
to recognize. The most common virus in greenhouses is impatiens necrotic spot
virus (INSV). While not all plants are hosts, the host list is at least 500
species long. The symptoms vary considerably from plant to plant but once you
can recognize INSV on a specific plant, you can usually identify it the next
time it occurs on the same host. Very reliable test kits that are easy to use
are available for testing INSV on-site.

Other viruses can produce distinct symptoms but, for the
most part, cannot be reliably diagnosed outside of the lab. Viruses should be
identified accurately because they are carried by different insect vectors and
have different host ranges. Environmental controls: Destroy symptomatic plants;
keep insect and mite pests under control; control weeds.

Diagnostic Kits and Services

Identification of diseases caused by fungi, bacteria
and INSV.
Plant
Disease Diagnostic Lab, Department of Microbiology, Fernald Hall, University of
Massachusetts. Call Rob Wick, (413) 545-1045.

Identification of viruses of all kinds style='font-weight:normal'>. Agdia Testing Services. 30380 County
Road 6, Elkhart, IN 46514. (800) 622-4342.

On-site test kits for Xanthomonas camptestris pv.
pelargonii, INSV and TSWV.
Agdia Testing Services. 30380 County Road 6, Elkhart, IN 46514. (800)
622-4342.

On-site test kits for Xanthomonas camptestris pv.
pelargonii, INSV and TSWV.
These kits were developed by Hydros Environmental Diagnos-tics
Corporation, Falmouth, Mass., and are available through Griffin Greenhouse
Supply, 1619 Main St., Tewksbury, MA 01876. (978) 851-4346.

Additional kits for Pythium, Phytophthora and Rhizoctonia are
available directly from Hydros Environmental Diagnostics Corporation. (508)
540-2229.

On-site test kits for Pythium, Phytophthora and
Rhizoctonia.
Neogen
Inc., 620 Lesher Place, Lansing, MI 48912. (517) 372-9200.

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This article was re-printed with permission from Floral
Notes, University of Massachusetts.

About The Author

Robert L. Wick is associate professor in the Department of Microbiology at the University of Massachusetts, Amherst. He can be reached by phone at (413) 545-1045 or E-mail at rwick@pltpath.umass.edu.

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