«VIRUS DISEASES OF GERANIUM Several diseases of geraniums (Pelargonium species) are caused by viruses. The economic losses caused by these diseases ...»
By far the most important method of spread of viruses is by stem cuttings used for vegetative propagation.
Since geraniums are propagated from relatively few source (“mother”) plants, the potential arises for virtually the entire plant population to be infected. Generally, geranium plants vegetatively propagated from virus-infected mother plants are infected.
Sap transmission of viruses from one geranium to another on cutting knives and by handling has proven to be difficult. Transmission is particularly difficult in the summer when the virus concentration in the host is low. Also, virus inhibitors may be more concentrated in the sap of virus-infected geraniums during the summer.
The relationships of insects as vectors in transmitting geranium viruses have not been determined. Many insects–such as aphids in the case of cucumber mosaic virus–known to transmit the viruses that infect geraniums have failed to transmit these viruses to geraniums. Some insect vectors will not even feed on geraniums. Geraniums in California fields harbor populations of whiteflies, leafhoppers, and aphids, so it does not seem unreasonable that insect transmission occurs, particularly in areas where stock plants for vegetative propagation are grown outdoors.
Control Large-scale control of geranium virus diseases can be divided into three major areas: (1) indexing to identify infected plants; (2) producing virus-free plants; and (3) maintaining disease-free plants.
A. Start with virus-free plants. It is imperative that florists obtain “culture-indexed” cuttings from a commercial propagator who has established virus-free planting stock and practices strict sanitary measures. Culture-indexed cuttings are used to set up an increase or mother block of virus-free plants. This is best accomplished using a combination of destroying (roguing) infected plants and periodic systematic testing of stock plants where virus or other diseases are suspected.
1. Virus-indexing is the most important step. Diagnosis of virus-infected but symptomless geranium plants may be accomplished by grafting to susceptible geranium cultivars or wild species of Pelargonium and by sap transmission to other herbaceous indicator plants such as species of Chenopodium, tobacco, lambsquarter, peas, French beans, petunia, and cowpea that exhibit reliable symptoms. It takes two years to produce suitable geranium tester plants from seed.
equipment are available. Viruses such as cucumber mosaic, TomRSV, and TRSV are most easily detected in young leaves, flowers, and ligula. The ELISA test is currently the most rapid, economical, and convenient method for commercial indexing and is more reliable than bioassay on indicator plants.
B. Viruses can be eliminated from a desirable variety by using various techniques. Some geranium clones from the same source may be virus-free. These can be selected to perpetuate the line.
Alternatively, shoot tips from rapidly growing stems may be virus-free, and geranium plants started between March and September from small tip cuttings will often be virus-free. This method is employed for the leaf curl or crinkle disease. Growing infected plants at high temperatures for prolonged periods has been used to produce virus-free carnations and chrysanthemums. Geraniums infected with leaf curl or ringspot and grown at 100°F (37°C) for 4 weeks produce small shoots with tips that are free of the virus(es). Meristem tip culture is another possible method of eliminating viruses from geraniums. It is also possible to obtain single, virus-free cells from a callus culture derived from an infected plant. Techniques for regenerating such geranium cells into an intact plant are largely in the future, however.
C. Geraniums free of viruses and other pathogens can be maintained by following strict sanitary
precautions to prevent contamination of healthy plants:
1. All infected plants should be immediately removed and destroyed, preferably by burning.
2. Florists should use an automatic watering system. If this is impossible, hang the hose so the nozzle does not touch the greenhouse floor.
3. When propagating, select a raised bench away from other areas where geraniums are produced.
The bench surface should be wiped or sprayed with a fresh household bleach (Clorox, Purex, Sunny Sol) solution, prepared by adding 1 part of liquid bleach to 4 parts of water.
4. Steam the propagating medium, soil mix, flats, clay pots, and tools for 30 minutes at 180°F (82°C) at the coolest spot.
5. Wash hands thoroughly with soap and hot running water before handling plants. Take cuttings from disease-free stock plants by (a) breaking out succulent tips or (b) sterilizing the cutting knife or razor blade by dipping it in 70-percent solution of rubbing alcohol and flaming.
Knives or razor blades should be changed between geranium stock plants.
6. Place cuttings in clean, sterile flats lined with fresh newspapers.
7. After establishment, take the cuttings from the flats and plant them directly into steam-treated soil mix in pots. The handling of plants should be kept to a minimum in order to avoid the spread of certain viruses and other pathogens by mechanical means.
8. Maintain good cultural conditions and ventilation. Eliminate all plant debris and unsterilized soil that could act as a source of contamination.
10. Avoid forcing plants too rapidly, especially during warm, humid weather when the temperatures are 70° to 85°F (21° to 29°C).
11. Maintain balanced fertility based on a soil test.