Diagnostics and pathogens of Kalanchoe and Pelargonium diseases

Major Kalanchoe diseases

Kalanchoe is susceptible to a number of dangerous diseases affecting the root system, stems, and leaves. Root and crown rots are caused by pathogens Pythium, Thielaviopsis basicola, and Rhizoctonia solani. Great damage to plantings is caused by powdery mildew, caused by the fungus Erysiphe polyphaga. Stems and leaf stalks are affected by gray mold, the pathogen of which is Botrytis cinerea.

Upon infection with Verticillium dahliae, plants develop stunting and premature aging. Kalanchoe is also affected by white mold of stems, caused by Sclerotinia sclerotiorum, which is easily recognized by its characteristic white, woolly mycelium and large black sclerotia. In addition, leaf spot may appear on the leaves, caused by a fungus of the genus Ascochyta. Few effective and proven methods of controlling this group of diseases are currently known.

Pelargonium diseases: diagnosis and protection

Pelargonium is one of the most popular potted and bedding crops, which suffers greatly from harmful infections. Black stem rot, caused by the soil-borne fungus Pythium splendens, affects plants at various stages. During cutting rooting, it manifests as soft, wet rot at the base of the cut. Later, the disease affects the roots and the base of the stem, which can be identified by the gray-green color of young leaves, wilting, and drooping of the plants. Adult specimens, when affected, lag in growth and mature prematurely before the rot reaches the soil surface. Root rots of pelargonium can also be caused by other pathogens: Phytophthora sp., Thielaviopsis basicola, and Myrothecium.

Bacterial rot, or leaf spot, caused by Xanthomonas pelargonii, is considered the most dangerous disease of the crop. P. zonales cultivars suffer most from it, and the latent period of the infection can last for several months. Symptoms of the disease are similar to those caused by Pythium splendens, but the rot is usually drier, and lesions can occur above the soil line, especially on old lignified shoots. When leaves are affected, chlorosis or small brown rounded spots with a water-soaked halo appear on them, the expansion of which leads to the death of the leaf blade.

The infection is easily transmitted by water splashes, but the main route of spread is the transfer of bacteria on hands and knives during care and taking cuttings. Contaminated tools can infect dozens of healthy cuttings and stock plants. The main method of controlling bacterial wilt remains the strict maintenance of the health of mother plants.

1. Immediately destroy any affected plants and replace them with healthy ones.
2. To preserve valuable clones, use a cloning system with a mandatory six-month check of all cuttings from the parent form. If at least one plant becomes diseased during this period, cull the entire clone.
3. Purchase ready-made healthy planting material from specialized firms for breeding mother plants.

Verticillium wilt, caused by Verticillium albo-atrum and V. dahliae, is most dangerous for P. regalis cultivars. P. zonale cultivars are less affected, and ivy-leaf cultivars also show susceptibility. The disease causes stunting, growth retardation, chlorosis of lower leaves, and a dull green color of the upper ones without pronounced vascular browning. Signs of the disease are most pronounced during flowering, but by the end of summer, affected plants may temporarily restore normal growth. The pathogen persists in the soil and is often spread via infected cuttings.

If verticillium wilt is detected on mother plants, they must be completely destroyed. Do not keep asymptomatic plants from affected batches for further propagation, as they are highly likely to contain a latent infection. Rust, caused by the fungus Puccinia pelargonii-zonalis, also occurs on pelargonium.

This disease, first discovered in 1962, spread rapidly across Europe. The pathogen affects P. zonale, causing Fig. 17.10. Verticillium wilt of pelargonium. Top — cultivar 'Rush Carpet', bottom — cultivar 'Grand Slam'; left — healthy plants, right — affected plants with signs of stunting and wilting of mature leaves, although the disease rarely leads to plant death. Fig. 17.11. Pustules of the rust pathogen (Puccinia pelargonii-zonalis) on the underside of a leaf; the central, larger pustule is surrounded by a ring of smaller pustules. Typical pustules on the underside of leaves and small chlorotic spots on the upper side. In severe cases, leaves wilt and die. Urediniospores are rapidly dispersed by air currents and water droplets; in a warm and humid environment, an epiphytotic develops.

Control. The most effective method of control is the use of healthy stock material. To stop the spread of the pathogen during a rust outbreak, it is necessary to maintain relative humidity at 80–85% or lower and avoid surface wetting of plants. Eradicating the pathogen is difficult. Affected leaves should be removed immediately whenever possible. Applications of dithiocarbamates, such as zineb or maneb, or one of the products with specific action (benodanil) help to keep the disease at a low level, and sometimes eradicate it completely.

Most often, this pathogen affects P. zonale. Watery spots with a brown center appear on the underside of leaves, which gradually enlarge and merge into large zones. On cultivars of P. peltatum, the pathogen develops mainly on the lower leaves. The symptoms of the disease somewhat resemble edema (growths). Fungal spores are dispersed by air and water droplets.

Control. High-volume spraying with zineb, performed regularly, is highly effective.

Other diseases developing on pelargonium are of less importance. For example, the availability of healthy mother plantations has significantly reduced the problem of viral diseases. In the past, great damage was caused by the pelargonium leaf curl virus, which severely affected the youngest leaves, especially in spring and early summer, and was often accompanied by various types of yellows. This was obviously due to mass infection of mother plants with a complex of viral diseases, which led to the appearance of deformations, mosaic patterns, ring spots, and yellows as a result of mixed infection.

Sometimes bacterial gall (Agrobacterium tumefaciens) and fasciation (Corynebacterium fascians) appear on pelargonium.

In the first case, woody galls form on the stems, usually at the soil surface or slightly above. Fasciation manifests as gall-like deformations with leaf-like outgrowths, mainly at the base of the stem. The affected plant dies. Very common on all types of pelargonium is gray mold, which develops on leaves and stems and causes flower spotting, especially on P. peltatum. This disease is easily controlled by managing environmental conditions and applying fungicides, which must be rotated to prevent the development of resistance in the pathogen population (see page 131).

During the winter months, growths appear on the leaves of pelargonium, especially ivy-leaved, which do not cause significant damage (Fig. 17.12).