Pests, Diseases and Disorders of Begonias

Few pests or diseases trouble begonias. Most can be guarded against by paying attention to the basic requirements of ventilation and humidity. If you choose to use chemical controls, manufacturer's instructions should be carefully observed, including the use of protective clothing.

Main pests

Vine weevil: The black vine weevil (Otiorhynchus sulcatus) is found throughout the world and in some areas causes severe damage to commercial crops. With the increased use of container plants, it has spread from nurseries and now flourishes in our gardens. Adult weevils are about 1/2 in (1 cm) long and black, with a number of lighter spots on the shell. The first adults emerge in early spring and continue to do so over a period of about two months. Although they are flightless, they are evidently excellent climbers, as larvae are often found in tubers of Pendula begonias hanging many feet above the ground. The larvae, which are legless, are about 1/4 in (7 mm) in length. They are crescent-shaped, white or cream in color, with a light brown head. Pupation occurs in mid- to late winter in a smooth-walled cell in a tuber, in the potting mix, or in open ground. The weevils enjoy a varied diet, with the adults feeding at night on the leaves of any of some 100 different plants, where they leave a characteristic notched pattern on the edges. But the larvae are the most destructive during late summer and fall, with their unobserved voracious attack on the roots and, in the case of begonias, on the tubers. The growing plant, under attack by these larvae, may wilt and not thrive, as the small roots so important to growth are being eaten. It pays to check a wilting plant for any signs of infestation by knocking it out to check the roots rather than applying more water immediately. A dormant tuber infested by the weevil larvae may have anything from one small hole to many holes, or may even be just a husk where the weevils have dined out in style. The most sensible way to combat all pests and diseases is with prevention, and this is particularly so with the vine weevil, since most controls are very toxic. Avoid growing begonias with plants such as cyclamen, which have a reputation as one of the weevils' favorite foods. Check the root ball of any growing Tuberhybrida purchased or received during the season, and be vigilant for evidence of activity when repotting tubers. Potential hiding places, such as plant debris and loose boards, are best kept clear of the begonia growing area. It may be possible to catch the adults by looking in such places, but they are shy and nocturnal, so a flashlight search is the only way to find them. Careful cleaning and inspection of lifted dormant tubers, with perhaps a dip in insecticide, will prevent the weevil larvae transferring from one tuber to the others in their storage area. When potting, fresh mix from a reputable source is recommended; the re-use of old potting mix should be avoided. As few as six larvae in a container can produce 6000 offspring in a season-thus, even one left in old mix can lead to problems. Most controls are intended for attacking the larvae and are mixed with potting mixes or in the soil to give protection throughout the growing season. The active ingredient in such products is chlorpyrifos, a contact and ingested organophosphorus-be warned that these are toxic chemicals, and any leftover mix cannot be used on the vegetable garden. Biological controls are also available in the form of nematodes. These microscopic creatures enter the bodies of the larvae, kill them, and continue to multiply, killing any new grubs that hatch during the growing season. The downside is their very short shelf-life and the large quantities they are marketed in, making it difficult for the home gardener to use them.
Mites: There are a number of different types of mite, some clearly visible with the naked eye, such as the European red mite, and others visible only with a microscope, such as the Tarsonemid or cyclamen mite. The latter are the most common type found on begonias. Consequently, their presence may not be noted until after the damage to the buds and growth tips, where they live, has occurred. Symptoms of a mite infestation are a cork-like substance on the stems of plants and the underside of leaves. The growing tips turn brown and become very brittle, collapsing when touched. Flower buds become distorted, and a brown stain is seen as the flower struggles to open. Plants can be decimated in a very short space of time, and the mites are often well-established before the symptoms are readily apparent. Mites thrive in hot, dry conditions, so growing under glass heightens the risk of an infestation. Both high humidity and good ventilation around the plants will help prevent infection. Other plants known to be sources of mites should not be grown in the vicinity of begonias; these include cyclamen and strawberries. A wise preventive measure is to isolate new acquisitions, regardless of the source. Note that insecticides are not effective against mites, and a proper miticide (acaricide) is required. Generally, the more lethal to mites, the more toxic the spray, so take full precautions when using the sprays. Low-toxicity sprays may prove effective, perhaps largely because of the mites' dislike of moisture. Seaweed products, which can also be applied as a foliar feed, keep the pests at bay, possibly because of the smell. Pest oil and wettable sulfur are other low- toxicity methods. Another non-toxic option is washing soda (sodium carbonate), using 1 teaspoon in 2 gallons (9 L) of cold water, plus a squirt of detergent. Plants can either be sprayed with this solution or totally immersed, if not too large.
Minor pests: Some other pests that may occasionally occur are root mealybugs, thrips, the green looper caterpillar and sciarid flies (peat gnats). These can generally be readily controlled with the use of any good insecticide.

Main diseases

Mildew: Powdery mildew is a fungal disease that attacks a wide variety of ornamental plants, including all begonias. The damage due to infection by the fungi can be slight to severe; affect some plants and not others; and be worse in some seasons than in others. Usually the first symptoms of the disease are white to pale gray fungus growths-similar to small white stars-which appear on the leaves, stems or flowers. Young plants and those under any stress, such as one that is badly in need of water, tend to be the first affected and more severely damaged than older, healthy ones. Generally speaking, mildew is more prevalent in the fall when high humidity is rapidly converted to moisture by the onset of cooler evenings. Moisture allows the spores to germinate. In a conservatory or greenhouse, adequate air circulation is vital, as is control of the humidity. Shade houses have the advantage of better ventilation, but jamming plants close together will be counterproductive, as it will reduce the all-important airflow around them. In the open garden, good spacing will reduce the chances of infection. Spraying and watering is best done in the early morning, with care taken not to wet the foliage when watering. An option in areas particularly mildew-prone is to consider adopting a preventive spray program from the beginning of the season. Before buying the spray, check the information given on the container for the following names, as these are the present-day chemicals supposedly best suited to controlling powdery mildew: proaconazole, myclobutanil, triforine, tridemorph, penconazole and fenarimol. In some instances, copper fungicides can also be effective. Sulfur, both as a spray or a dust, is also effective and is now available as a soluble powder. As opposed to all other sprays, mildew does not become resistant to sulfur. One old remedy, for those who prefer not to use toxic sprays, is to use 2 tablespoons of baking soda to 1 gallon (4 L) of water applied as a spray. Another natural product one can use is milk. In an experiment carried out in South America, it was found that milk diluted with ten parts of clean water and sprayed once a week was very effective in controlling powdery mildew on vegetables.
Botrytis (stem rot or brown rot): Botrytis is a fungal disease that can attack any part of a begonia plant, particularly in the latter part of the season, as does mildew, since it is associated with the same conditions that favor the development of mildew. Botrytis is evidenced by the appearance of a wet, soggy, brownish area which, if left untreated, will quickly increase in size and may eventually kill the plant. In the tuber, botrytis usually results from damage or failure to remove the scab. Another form of botrytis that affects tubers in storage is dry rot, when they go hard for no apparent reason and die. Botrytis usually affects plants, or parts of plants, that have either been damaged or have insufficient ventilation, or where the grower's hygiene is poor. Examples are a tie cutting into the stem of the plant, or the plants being too close together, or falling flowers becoming lodged in the foliage below. Excess nitrogen will also predispose a growing plant to botrytis because the new growth is soft. Good hygiene, adequate ventilation, and plant spacing are of paramount importance in preventing the onset of botrytis. Pay particular attention to removing stems and leaves as they drop, for they will cause the infection to start, when left lying on other parts of the plant. If any area of rot is found, it must be dealt with immediately, especially if it is close to the tuber. The first step is to clean a knife or scalpel by dipping it in methylated spirits. If the stem is rotten right through, then the only option is to cut it off cleanly with the sterilized knife below the infection, which may mean the end of that particular plant's growth for the season. If the stem is only partly infected, then the area can be cut away carefully, ensuring that all the rot is removed. The cutting instrument should be cleaned both during and after use. Once all the rot is removed, wash the wound with methylated spirits, which will both sterilize and dry it. Then dust the wound with either flowers of sulfate or another anti-fungal powder. This will have the dual effect of drying off the wound and sealing it from possible further infection.

Disorders

Foliar petal: Foliar petal is a condition where green, leaf-like petals replace the actual dorsal or guard petal of the flower buds, giving the appearance of a leaf rather than the normal petal. This often happens on the first few buds of the season, with subsequent flowers being normal, and is more prevalent on the paler white and yellow varieties. The condition is not caused by any pest or disease but is thought to result from overfeeding-particularly with nitrogen-in the early stages of the plant's growth, although there is no scientific evidence that this is so. Any affected buds should be removed.
Corky scab (edema): This condition gives the appearance of a pale brown, cork-like scale on the plant stems and the underside of young leaves. It should not be confused with marking left by mites-corky scab is coarser and covers areas rather than appearing in the form of lines. The cause is often over potting. When a young plant is over potted, its small root system is unable to cope with the amount of moisture available in the mix. The leaves try their best to transpire this moisture, but there is so much that their cell structures burst, leaving the brown corklike appearance. In turn, since the plant can now cope with even less moisture because of the damaged leaf cells, the soil becomes even more waterlogged, resulting in the actual drowning of the roots. No oxygen is able to get into the mix, which in turn reduces the plant's ability to function. If continuously exposed to excess moisture, the roots will rot, causing the plant to die.
Bud drop: The dropping of buds as they form may occur if the plant is under some form of stress, such as excessively high temperatures, especially if accompanied by low humidity. Over watering or rapid changes of temperature are other causes. Large-flowered Tuberhybrida can make difficult houseplants because of the lack of both ventilation and humidity, so bud drop is common when plants are taken indoors for more than a day or two at a time. Outside, bud drop is not so prevalent, although it will occur in hot, dry conditions. Every effort must be made to keep the humidity high, with pebble trays filled with water or humidifiers in conservatories, or by wetting the floor in the shade house or greenhouse in the morning or early afternoon.