October 2002

Grower 101: Diagnosing Plant Diseases of Floricultural Crops By Robert Wick

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. Of all of the diseases,damping-off and root rot are the most difficult to identify on-site, most oftenbecause wilting, stunting or nutrient deficiency in the leaves distract growersfrom the root system that is causing the problems. Pythium and Rhizoctonia arethe two most common causes, but they usually do not leave any clues as to theiridentity. Also, they have a wide host range, so the plant on which they occurdoes not supply a clue. When Rhizoctonia is causing web blight, you can see thebrown strands of mycelium on the diseased plants. Webbing occurs whentemperature and humidity are high. There are test kits available that can allowyou to make an accurate on-site identification of Pythium, Phytophthora andRhizoctonia. Environmental controls: Destroy entire plant and its soil; improvemedia drainage; use new containers; plant at appropriate depth.

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 blight of geranium iscaused by Xanthomonas. Wilted plants will not have root rot or stem canker(unless there are several diseases occurring). At first, one or two leaves willbecome soft and droop. Root rot or high soluble salts can also cause wilt butusually the whole plant wilts. Another good indication of Xanthomonas is leafspots, but leaf spots do not always occur. Ralstonia (Pseudomonas) solanacearumcan also cause a systemic wilt of geranium but does not cause leaf spots. Werarely see Ralstonia in geranium in New England, but we had several cases in1999. Xanthomonas only infects geranium but Ralstonia infects a variety ofornamentals and vegetables (as well as tobacco and banana).

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. There are many fungi that causespotting of floricultural crops. With experience, some can be recognized butmany cannot be identified on-site. Alternaria on zinnia, impatiens and gomphrenais relatively easy to identify. Botrytis is easy to identify when it isproducing its crop of gray spores. On fuchsia, Botrytis causes cankers, whichusually do not develop spores, but cankers on fuchsia are almost always causedby Botrytis. Environmental Controls: Remove damaged leaves when plants are notwet and destroy leaves; use drip irrigation; space for good air circulation.

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. Diseases caused by these fungiusually affect only part of the plant at first. Foliage on the affected stemswill wilt and usually become yellow. There may or may not be discoloration inthe vascular tissue (cutting into the stem will reveal a dark stain). It isdifficult to diagnose these diseases with much confidence in the field. Withcyclamen, several leaves will turn yellow. When you cut through the corm (cutthe top off) you will see a dark band of vascular tissue that corresponds withwhere the yellowed leaves were. These two vascular wilt fungi are not commonlyseen in greenhouses. When they do occur, it is usually on chrysanthemum, cyclamenor basil. Environmental controls: Destroy entire plant and soil; improve soildrainage; use new soil.

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.