Diseases, Pests and Control

by Harriet Olds

Reprinted from Points of Interest, Newsletter of the Colorado Cactus & Succulent Society

Diseases and pest infestation of cultivated plants are frequently the consequences of unfavorable growing conditions. Plants weakened by temperatures running too high, lack of fresh air, poor light, and excessive amounts of nitrogen are particularly susceptible to disease.

Nonparasitic diseases are always due to errors in cultivation. In spite of all of the efforts to create good conditions for growth, diseases and pest attacks cannot be eliminated completely. Sometimes they are brought in from outside or transmitted by other external influences from adjacent areas. The symptoms are frequently very serious, threatening the existence of the plant, so the use of highly efficient pest-control agents is unavoidable.

The following important points apply to every pest-control measure:—Use a suitable preparation that is tolerated by the species of plant in question—Time applications correctly—Comply with directions for use—Apply the correct amount—Allow for temperature variations

The basic rule is: Prevention is better than cure. Treatment must therefore always be commenced in good time. Once pest-control measures are begun, they should, as a rule, be repeated several times within a few days, since often only the larval stages, and not the eggs, are treated (e.g., red spider mite, white fly).

To prevent possible resistance to certain agents, vary the preparations or active ingredients used to control the pests. Pest-control chemicals should always be applied conscientiously, to protect one's health, prevent damage to the plants, and minimize environmental pollution. When applying a chemical in a water solution, be sure the water itself is not cold. Chemical solutions should be applied on a warm day but not in full sun, and don't apply them to dry root balls.

Appropriate cultivation practices are your first line of defense against disease and pest attack. Grow cacti and succulents to be as healthy and resistant as possible with plenty of light, fresh air, balanced feedings, and pronounced temperature differential to harden them off. They need to be checked thoroughly at regular intervals for disease and pests. Pay particular attention to any plants that haven't shown new growth and don't produce new spines in the growth period, that have unexpectedly refused to flower, that are not healthy and fresh in body color and appearance, and that are still not firm and secure in their pot even after a fairly long period of growth. Newly acquired plants should not be placed with the rest of the collection until they have been thoroughly examined, appropriately treated, and kept in quarantine. Don't throw diseased plants on the compost heap or in a corner of the potting bench, where they could become a breeding ground for generations of pests and dangerous fungus spores. Cleanliness should prevail in the cultivation premises. Always wash yourself carefully after using plant chemicals and clean all the equipment you have used.

In the following list, the most disastrous diseases and pests and the damage they cause will be described, and possible methods of treatment will be given.

Fungal diseases

Rhizoctonia rot in seedlings and cuttings causes plants to turn transparent in appearance. The wet rot proceeds from the stem base of the plant and is recognized further by its fine web of thin white/gray threads spun over the soil.

Control: Captan 50, Orthocid 50 (each 0.1?0.2%); Chinosol (0.05%); Benomyl (0.05%). In the case of sensitive seedlings, the plant compatibility is sometimes unsatisfactory. The consequence can be: reduced or retarded germination and inhibited growth. Plants affected by this fungus should be carefully removed and less-severely-attacked seedlings should be pricked out into a disinfected (preferably steam-treated substrate.

Helminthosportium stem rot is shown by the mummy-like shrinkage of young plants. This disease is brought in with seeds from the native countries of the cacti. The infestation spreads very rapidly and can cause the total loss of a whole sowing within a few days. The disease occurs only at low temperatures in combination with high atmospheric humidity. The fungus is easily recognizable by the velvety-green spore coating.

Control: The plants affected should be removed immediately and healthy seedlings transplanted. Spray at ten-day intervals with Orthocid 50 (0.2%) or Pormasol forte (0.2%). The most important preventative measure is disinfecting the seed.

Epiphyllum mosaic disease is common among cacti. The symptoms are pale yellow, slightly sunken spots that are not clearly defined and appear predominately on the edges of parts of the plant and spread toward the center. Diseased plants are very reluctant to flower. Schlumbergera, Zygocactus, and Rhipsalidopsis are easily attacked. Diseased specimens should be destroyed. Fusarium rot and wilt is especially common in tree cacti but it also effects columnar and globular cacti. This fungus frequently attacks at the neck of the root. The tissue there turns brown and sinks in. The fungus then proceeds from the roots via the vascular system to the tips of the shoots, causing wilting. If a shoot is cut open, the typical characteristic of the disease can be seen-a red-brown coloration of the vascular bundles. Affected roots are destroyed completely so that only the fibrous vascular cords remain. The affected parts that are above ground are very soon coated with reddish, salmon-colored or violet spores. The infection mostly occurs at points of injury, often due to insects having bitten or eaten a part of the plant.

Control: Use only steam treated soil; remove any plants suspected of infestation; prevent injuries of any sort; keep temperature and humidity as low as possible; avoid excess application of nitrogen fertilizers. Chemical control with Do Pont Benomyl (0.05%), applied by watering or spraying.

Pythium and Phytophthora cactorum are wet-rot pathogens that come from contaminated soil and usually attack the stem base. The damaged plant becomes pulpy and its tissue rots.

Control: Disinfect soil; destroy affected plants and their pots. Water with Prothiocarb-Previcur (0.15%) or with Fenamisul-Bayer ( 0.03%). Successes are also achieved with systemic soil treatment, using Dexon, which can be sprinkled or watered on. Moisture promotes the spread of the stem-based rot; therefore-in cool weather at least-watering should be restricted to the minimum requirement.

Anthracnose disease symptoms are circular, sunken, pale, or brownish spots, where the plant tissue dries up and becomes hard and bark-like.

Control: In the initial stage, the diseased parts can be cut back to healthy tissue using a knife that is repeatedly disinfected by passing it through a flame or dipping it in medicinal alcohol. Chemical control by spraying with Saprol (0.15%), Maneb powder spray (0.2%), Dithane, or Orthocid 50.

Black Spot, a horrible fungal disease, predominantly occurs when the humidity is too high during the cool seasons of the year.

Control: Do not subject plants needing warmth to temperatures that are too low; reduce humidity. Chemical control with Benlate, and Orthocid 50, and Captan combined with Benlate is effective as well.

Treatment of most fungus attacks calls for the affected plants and all the soil in the pot to be destroyed. Thoroughly examine plants that appear to be healthy, and wash all of the soil out of the roots. The next stage is to make up a fungicidal solution and submerge the entire plant in it for half an hour (A combination of Previcur 0.15% and DuPont Benomyl 0.05% is effective against many species of harmful fungi.). After soaking the plants, they must be allowed to dry out for five to ten days, and then they should be planted in very permeable soil (with a high proportion of pumice or lava-rock gravel, perlite, and similar substances). If one of the salvaged plants again shows signs of fungus attack the plant will need to be soaked in a mixture of Previcur and Benomyl several times at fourteen-day intervals. Hardened plants that have been correctly cultivated have a high resistance to fungus attacks.

Viruses

Viruses and mycroplasms are minute pathogenic organisms that cause a variety of symptoms in plants. Plants with virus infections cannot be cured, not even can parts of the plant be rescued. Chemical control is not yet possible. The disease is transmitted in the sap of affected plants by insects that pierce the plant body or from a grafting knife that has not been disinfected. Stunted growth and the pathological, super-abundant shoot growth of Opuntia monstrosa and monstrous-dwarf forms of some other opuntias are the symptoms of a developmental disorder caused by viruses. These strange and unique forms are consciously propagated and kept in plant collections. Insect pests

Eelworm or nematode are considered extremely dangerous and difficult-to-control insect pests for cacti and numerous other succulents. Of the latter, crassula and geranium can be very severely attacked. There are three subgroups of eelworm:

1. Root-knot eelworm. The larvae, which are about 0.5 mm long, bore into the roots, damage the plant by sucking the sap, and cause knot-like swellings due to secretions from their saliva glands; the swellings quite frequently reach the size of anything from a walnut to a fist.

2. Cyst-forming eelworm. These pests live in the roots. They burst the root epidermis, and then the females cling to the outside of the roots as brown, lemon-shaped cysts.

3. Free-living nematode. These eelworm do not settle in particular places but migrate from plant to plant by their own movement, or they can be are carried by water. A single contaminated plant can quickly infest an entire collection. Nematode are about 1 mm long, colorless, and worm-like in shape. An eelworm attack is often not noticed until the plant begins to look stunted from considerable damage to the roots. Eelworm multiply rapidly, and their larvae can become dormant for months or even years, if their living conditions are unfavorable. These persistent pests may also be spread by careless handling of infected plants, pots, and boxes; by contaminated soil, greenhouse staging, and tools; or even by the soles of shoes.

Control: Extremely careful hygiene is required to check a plague of nematode. The simplest method of disinfection is to scald the plant containers, greenhouse benches, and tools with boiling water, but not all plastic pots and boxes will survive this treatment. Chemical decontamination of these objects can be performed with Formalin or Nemafos. The soil may be decontaminated by heating (steaming) or with Basamid. Nemafos emulsion can be used to water growing plant stock, but unfortunately, this preparation is not universally tolerated by epiphyllum and other epiphytic cacti. In combating Nematode, all the mechanical and non-dangerous methods should be tried first. For example, it is best to remove all severely affected roots when transplanting. Sometimes there is no choice but to cut off all of the roots and root the plant anew in fresh substrate. This can be done without difficulty with species of echeveria, crassula, kalanchoe, and sedum. Eelworm are sensitive to heat and can be killed by giving the plant a 30-minute bath in water at a temperature of 45?50 degrees C. The cactus roots should be able to stand this temperature. This kind of treatment, however, is also considered risky by some growers.

Spider mites are minute brownish-red mites hardly visible to the naked eye. They cover the affected plant parts with a fine white web. Spider mites often occur in vast numbers. They multiply rapidly in dry air and warmth and cause damage by piercing the epidermis and sucking the sap. The characteristic signs of attack soon appear as brown, gray, rusty and sometimes scabby areas of tissue which can spread over the whole plant. The attack begins in the region of the apex on globular cacti. The cacti most at risk are rebutias, lobivias, various mammillarias, aporocacti, and Chamaecereus silvestrii; the succulents at risk are crassula and faucaria. Once the epidermis has been destroyed by an attack of spider mite, it never regrows. It is a long time before affected areas are at least partially covered by new growth.

Control: Growers who cultivate their cacti hard and who find large numbers of real spiders of all sizes in their collections only rarely suffer from a spider-mite attack. This biological balance can be preserved when red spider mite gets out of control by using Tedion V18 (0.02%) or Shell-Torque (0.05%). Neither of these preparations kills the winter eggs of the mite, but they both eliminate the larvae and mobile stages while sparing useful insects and the mite's natural enemies. Mite-killing preparations (acaricides) such as Dimethoate emulsion, E 605 forte, Kelthane, or Malathion are effective. To prevent the development of resistance to certain active agents, apply these preparations alternately. The treatment should be repeated two or three times at intervals of eight to ten days.

Scale Insects occurring on cacti and other succulents are predominantly in the form of mealybugs. Scale insects occur on agaves, leaf cacti, pereskias, and some other genera. Most species of scale insects reproduce by means of eggs, some are viviparous. The flat hemispherical armor of the disc-scale insect consists of wax-like integument or hardened back skin. In the juvenile stage, the insects are mobile and have legs, feelers, and eyes. The males have wings. Females shed their skin many times before eventually losing nearly all their organs, becoming immobile, and settling in one place. Their protective shells are then firmly fixed to the host plant. After depositing eggs, the female dies and several hundred eggs begin their rapid development, protected by the shell left behind. A frequent attendant symptom of scale-insect attack is the formation of sticky honeydew, followed by the growth of sooty mold.

Mealybugs exude a white waxy substance that envelopes their entire pink-colored bodies. Unlike the armored-scale insects, mealybugs remain mobile throughout their lives. They also multiply rapidly. Their sap-sucking activity weakens and eventually kills severely infected plants. If not controlled, mealy bugs will very soon form large colonies in protected corners of cluster-forming cacti like rebutia, mammillaria, dolichothele, and echinocereus. They will thrive in the axils where they are particularly indistinguishable from the axil hairs of some species like mammillaria. They will also locate themselves at the apex or on the stem base of species like echevera, crassula, faucaria, pleiospilos and other succulents.

Control: Cacti should be continually inspected for signs of this pest's attack. Upon inspection, if only a few mealybugs are found, it is sufficient to dab them directly with an ethyl alcohol solution on a brush or cotton swab. Another method is to spray them off with a jet of water using a fine spray nozzle. The use of chemical control methods is unavoidable in an advanced stage of infestation. Use a preparation specifically for scale insects or mealybugs (Malathion 0.1%, Metasystox, Dimethoate, Basudin liquid, or Roxion) because these insecticides adhere more readily to the water-repellent wax secretions. If you use a spray, thoroughly spray concealed places on the plant. The treatment must be repeated several times at weekly intervals.

Root mealybugs are among the worst pests of cacti and other succulents. They live almost exclusively on roots and the parts of the stem that are below the surface. They are similar to mealybugs in that they also secrete whitish, woolly, or powdery wax. Root mealybugs prefer dry substrates. They multiply particularly rapidly during the dry winter dormancy period. Affected plants look pale, become sickly, and gradually die. Plants damaged and weakened by root mealybugs are especially susceptible to fungal diseases.

Control: Brush off the bugs with a stiff brush when transplanting. Dip the roots in insecticide, and transplant into new soil. Water in the spring and autumn with suitable chemical solutions like Diazion 25, Metasystox R 0.1%, or E 605 forte 0.35%. It is advisable to leave the pots up to their rims in the solution for several minutes to ensure that the they soak up the liquid thoroughly from the bottom. If this is not possible, thoroughly drench the potted plant so that the liquid runs out the drain hole of the pot. Repeat the treatment several times at intervals of two weeks.

Aphids occur frequently on the flower buds of epiphyllum and yucca, and on the plant bodies of pereskia, senecio, and other genera.

Control: Slightly toxic insecticides are effective. Care should be taken not to spray insecticide into the open flower of epiphyllum, since this will ruin it.

The white fly or scale moth attacks leaf-bearing plants almost exclusively; e.g., pelargonium, Pereskia Fouquieria, crassula and senecio. This is a pest that occurs in great numbers and is difficult to control.

Control: Special preparations must be applied at intervals of three to four days. Actellic, Alphos mister, Ambush, Dichlorvos, and Parathion have proved successful. Use preparations in alteration. White fly settles on weeds such as chickweed, stinging nettle, frenchweed, greater celandine, and others, then moves on to cultivated plants. The control of weeds in surrounding areas is therefore an important preventative measure.

Fungus gnats are black insects 3?5 mm long, that lay their eggs in humus or decaying organic matter. Their larvae cause damage by eating seedlings, tender young plants, and young roots. Places where they have fed provide a foothold for Fusarium rot. The larvae are 6?7 mm long with translucent, glassy white bodies and black heads.

Control: A preventative measure is to use inorganic sowing medium. Otherwise, many insecticides are effective but should be used with caution due to the sensitivity of tender, young plants. Take care that preparations can be tolerated by the plants for which they are intended.

Excerpts taken from Cacti and Succulents by P. Perl, published in the June 1990 "Points of Interest" and from The Encyclopedia of Cacti by Cullman, Goetz and Groener, 1986, English edition.