Crown Rot is caused by the Phytophthora fungus, which attacks the roots and, most notably, the crown of African Violets. African Violets are most susceptible to Crown Rot when allowed to sit in soil that is heavy and soggy. In almost all cases, Crown Rot is fatal. The best control for Crown Rot is, therefore, prevention.
Distinguishing Symptoms
- Crown is mushy. It may appear translucent brown with a soft, jelly-like consistency.
- Crown darkens. It appears brown or black in color.
These symptoms indicate that the crown is rotting. If your African Violet has these symptoms, it probably has Crown Rot.
Other Symptoms
- Crown has a weblike substance on it. (Note: This substance should not be confused with the web left behind by Spider Mites, which clings to the flowers and plant hairs of the leaves. Nor should it be confused with the "cotton" specks which characterize Leaf Mealy Bugs.)
- Leaves have a weblike substance on them.
- Leaves darken. They appear brown or black in color.
- Leaves wilt. They may appear translucent brown with a soft, jelly-like consistency.
- Plant growth is slow.
BIOLOGY
Phytophthora species are soil-inhabiting pathogens that are favored by wet conditions. Although previously considered fungi, Phytophthora species are now considered to be in a separate classification called oomycetes. Species of Phytophthora produce resting spores that survive for years in moist soil in the absence of a suitable host. However, if the soil is completely dried out, these spores are less likely to survive for more than a few months. When a host is nearby and free water (water in soil pore spaces) is present in the soil, resting spores germinate to produce motile spores that can directly penetrate roots, branches, or crowns as long as free water is present. Wounds are not required for infection. Resting spores, decaying host tissue in the soil, and active cankers (disease-infected dead, sunken lesions in plant parts) can all be sources for new infections. The pathogen can be spread in splashing rain or irrigation water, in surface irrigation, and runoff water, and by movement of contaminated soil, equipment, or plant parts. Flooded and saturated soil favors the spread of Phytophthora to healthy plants.Some Phytophthora species are favored by warm weather, some by cool weather. Root rots of avocado, citrus, and tomato are favored by warm conditions, developing most extensively in late spring and early summer. Decay of crown, trunk, and branches of other tree species are favored by cool, wet conditions. These decays develop most rapidly in late fall and early spring.
Treatment
The most important factor in reducing the threat of Phytophthora rot is good water management. Avoid prolonged saturation of the soil or standing water around the base of trees or other susceptible plants. Irrigate only as much and as often as necessary; in an orchard, keep track of the soil moisture around each tree and water only when necessary. If you irrigate trees with sprinklers, use low-angle sprinkler heads and splitters to avoid wetting the trunk and lower branches. If using a drip system, place the emitters at least a foot away from the trunk. Avoid planting susceptible species on poorly drained or shallow soils. Water stress and/or salinity make some plant species more susceptible to infection when wetted subsequently by irrigation or rains.For all vegetable and orchard plants, provide good soil drainage. Good soil drainage is best provided before planting. Drainage should be plentiful to the rooting depth of the plants, generally 3 to 6 feet for trees, 2 to 4 feet for shrubs, and 1 to 2 feet for bedding plants. During favorable weather you do not want the roots and crown of a plant to remain wet for the 4 to 8 hours that are required for Phytophthora to infect the plant.Provide adequate drainage by breaking through soil compaction and hardpan. In poorly drained soils, or in an area where you know Phytophthora is present, consider planting trees and shrubs on mounds. The mounds should be 8 to 10 inches high. Planting depth after settling should be no deeper than as received from the nursery, with the upper roots near the soil level and the graft union well above the soil line. Do not install irrigated turf around the base of trees, remove all weeds, and do not water the crown area directly. Never cover the graft union with soil or mulch. If you are not sure where the graft union is, ask someone at the nursery to show you and mark it. Raised beds provide good drainage in vegetable garden situations also. Group plants according to their irrigation needs. Separate those needing frequent, light irrigations, such as potatoes and strawberries, from those needing infrequent, deep irrigations, such as tomatoes and melons. At the first signs of aboveground symptoms, examine the tree at the soil line for crown rot. Carefully cut away bark that looks affected. If crown rot is present, trees can sometimes be saved by removing soil from the base of the tree down to the top of the main roots and allowing the crown tissue to dry out.
Sanitation
It may be possible to slow the spread of Phytophthora within an orchard by avoiding movement of infested soil, water, and plant parts from an area where Phytophthora rot has developed. Surface and subsurface drainage water and anything that can move moist soil can carry the pathogen to a new area, including boots, car tires, and tools. If the physical setting allows drainage water to flow from infested to uninfected areas within the garden during wet weather, consider putting in drains to channel the water away from healthy plants.
Selection of planting stock
Plant only certified nursery stock from a reputable source, and choose the most resistant rootstocks or varieties available for your area. Less susceptible rootstocks or varieties are available for almonds and stone fruit, apples, cauliflower, and strawberries. Carefully select individual plants that are free of symptoms and/or that come from healthy lots of material.
RotationIf tomatoes have been affected by Phytophthora root rot, avoid planting tomatoes or other susceptible plants such as eggplant or peppers in the same soil for at least one or two seasons. Plant a resistant crop such as corn instead, or leave the soil unplanted and do not irrigate, but keep it well worked to allow the soil to dry as deeply as possible. Different species of Phytophthora attack beans and cole crops, so these plants can be substituted as well. Consult a nursery or farm advisor for possible alternatives in your area.
Chemical control
The most effective way of preventing Phytophthora rot diseases is to provide good drainage and to practice good water management. Along with the appropriate cultural controls, the fungicide fosetyl-al (Aliette) may be used on a number of ornamental plant species to help prevent Phytophthora infections. When applied as a foliar spray it is absorbed by foliage and moves into roots. However, do not rely on fungicide applications alone to control root and crown rot diseases.
Prevention
Prevention is always your best bet for successfully controlling Crown Rot. By adhering to the following guidelines, you will give your African Violets the best chance for eliminating any threat of Crown Rot.
Do not overwater. This is the greatest danger to African Violets. Overwatering leaves the soil soggy, a condition that is ripe for the development of Crown Rot. Water just enough to keep the soil damp. Always allow any excess water to drain. If you are using a self-watering device, which employs capillary wicking, make sure the pot is not in direct contact with the water. Also, it is strongly recommended that you only use wicks supplied by the manufacturer of the watering device.
Do not let the soil dry out completely before watering. Repeated, sudden changes in the availability of water can cause the same effects as overwatering.
Do not use a pot size that is larger than what your African Violet needs to form a tight rootball. If the pot size is too large, your Violet will not be able to absorb all the water, leaving the plant susceptible to the soggy conditions in which
Crown Rot thrives.
Make sure the pot you use provides adequate drainage. If you are using a plastic pot which has no holes, you can easily add holes yourself with a soldering iron or by simply heating up a screwdriver and pushing it through the bottom of the pot. If you are using a terra cotta pot which has no holes, you can add holes with a drill. For 4-inch pots, use a 1/8-1/4 inch bit and drill from the bottom. Do not press too hard. Go slowly and let the drill do the work.
Use a light, porous potting soil. An ideal potting soil will consist of block-harvested, sphagnum peat moss. Such a potting soil allows any excess water to drain away from the roots. Be sure to pasteurize your potting soil.
Maintain good air circulation around your plants. Keep your growing area clean, and remove spent flowers and leaves as you find them.
Finally, you should strongly consider using a recommended self-watering device. Such devices use capillary action to draw the right amount of water into the soil. This prevents both overwatering and underwatering. For miniatures, there is the MiniWell (fits 1-inch pots) and the Optimara WaterShip (fits 2-inch pots). For larger African Violets, there is the MaxiWell (fits 4-inch pots) and the Watermaid (fits various sizes up to 6 inches.) Each of these self-watering devices is available at a number of hardware and discount stores. In addition, they can also be ordered online from the Selective Gardener, a mail order supplier that specializes in plant care products for African Violets.
Important Note on the Use of Pesticides
Please note that almost all pesticides are formulated for specific uses and conditions. When applied incorrectly, pesticides can cause ill health or damage to plants. Therefore, when using any kind of pesticide or chemical treatment,
always apply as indicated on the product label.
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