Grower 101: Diagnosing Plant Diseases of Floricultural Crops
An accurate diagnosis of disease is important for several reasons.Bacterial diseases are not controlled with fungicides, and some bacterialdiseases are easily mistaken for fungal diseases. Some fungicides have a narrowspectrum of activity. In addition, if you know the disease, you can usuallyfind information regarding the environmental conditions necessary for diseasedevelopment. You could also find out if the pathogen was seed-borne orsoil-borne and whether other crops in the greenhouse may also be susceptible.
The ability to make an accurate diagnosis on-site isdependent on a disease that has unique symptoms. Also, the grower needs to havepreviously identified the problem or have a good illustration or writtendescription to make proper identification. There are a number of diseases thatcan be easily identified on-site, and there are many that can only be diagnosedin a university or private diagnostic lab.
Damping-off and Root Rots.
Soft-Rots. Cuttings have a large wound on the base and can be verysusceptible to Erwinia soft rot, especially when temperatures are in the 90sand there is plenty of water. When Erwinia is the cause, the base of thecuttings 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 bythe fungi Rhizopus choenephora and the bacterium Erwinia. These can bedifficult or impossible to identify on-site, but there are no fungicides orbactericides that will help. Simply discard the plants.
Bacterial Blight of Geranium.
Bacterial Leaf Spots. Bacterial leaf spots can look very similar to fungalleaf spots but when you hold them up to the light (as if to see the lightthrough the leaf), bacterial spots usually have a translucent look to them. Ongeraniums, you may see a halo around the spot. Bacterial spots on impatiens canbe distinguished from Alternaria leaf spot because Alternaria causes the leafto become yellow. Pseudomonas syringae and P. cichorii cause leaf spots ongeranium. Spotted leaves will look similar to those caused by Xanthomonas, butthe leaves will not wilt. Geranium leaves will turn yellow and dry up afterbeing infected but will not wilt. One way to distinguish bacterial leaf spotsfrom fungal leaf spots is to consider the environment. Bacteria usuallyflourish under hot temperatures; fungi prefer cooler conditions. Environmentalcontrols: Remove damaged leaves when dry and destroy leaves; keep plants as dryas possible; use drip irrigation; provide adequate spacing.
Fungal Leaf Spots and Blights.
Powdery Mildew. This disease is easy to identify because of the powderycrop of spores it produces on the leaf surface, which makes most any infectedplant unsaleable. In most cases, powdery mildew develops on the top side of theleaf and has a similar appearance on host plants, but on poinsettia and a fewother plants, mildew will also grow on the bottom. Powdery mildew rarely killsplants but can result in lower leaf drop. Occasionally, plants will develop purplish discolorations as a result ofsevere infestation. African violet, begonia, dahlia, gerbera, hydrangea,kalanchoe and pansy commonly develop powdery mildew. On kalanchoe, powderymildew can be difficult to recognize because only a fine webbing will develop.Environmental controls: Use resistant varieties; avoid crowding plants; growplants 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 andpale spots on the upper Á leaf surface. The pustules contain masses ofrusty to orange-colored spores. Snapdragon, fuchsia, carnation and geranium areamong the plants susceptible to rust diseases. To diagnose rust diseases, rubinfected leaves on a sheet of white paper; the rust-colored streaks left behindby spores are diagnostic for rust. Environmental controls: Remove damaged leaveswhen plants are dry and discard leaves; keep plants dry; avoid crowding plants.
Fusarium and Verticillium Wilt.
Viruses. Virus diseases can be either very distinctive or impossibleto recognize. The most common virus in greenhouses is impatiens necrotic spotvirus (INSV). While not all plants are hosts, the host list is at least 500species long. The symptoms vary considerably from plant to plant but once youcan recognize INSV on a specific plant, you can usually identify it the nexttime it occurs on the same host. Very reliable test kits that are easy to useare available for testing INSV on-site.
Other viruses can produce distinct symptoms but, for themost part, cannot be reliably diagnosed outside of the lab. Viruses should beidentified accurately because they are carried by different insect vectors andhave 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, bacteriaand INSV. PlantDisease Diagnostic Lab, Department of Microbiology, Fernald Hall, University ofMassachusetts. Call Rob Wick, (413) 545-1045.
Identification of viruses of all kinds. Agdia Testing Services. 30380 CountyRoad 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-ticsCorporation, Falmouth, Mass., and are available through Griffin GreenhouseSupply, 1619 Main St., Tewksbury, MA 01876. (978) 851-4346.
Additional kits for Pythium, Phytophthora and Rhizoctonia areavailable directly from Hydros Environmental Diagnostics Corporation. (508)540-2229.
On-site test kits for Pythium, Phytophthora andRhizoctonia. NeogenInc., 620 Lesher Place, Lansing, MI 48912. (517) 372-9200.
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This article was re-printed with permission from FloralNotes, University of Massachusetts.