Diagnostics and prevention of diseases in Peperomia, Pilea, and Poinsettia

When growing ornamental-foliage peperomia in a greenhouse, agronomists often encounter rots and leaf spots that quickly ruin the commercial appearance of the plants. Leaf and stem rot, caused by the fungus Phytophthora sp., is particularly dangerous. Under conditions of high humidity and heat, this infection manifests as soft, wet black spots, affecting both young cuttings and adult specimens. Among other threats to peperomia cuttings, the fungus Rhizoctonia sp. stands out, affecting the root system and foliage.

During diagnosis, it is important to distinguish the signs and pathogens of leaf spots on peperomia to adjust the microclimate in a timely manner. Some of them develop only on weakened or mechanically damaged plants, while others quickly destroy healthy cuttings. Below are the main pathogens and the characteristic manifestations of the diseases:

• Myrothecium roridum — a secondary parasite that affects aging or damaged tissues; its spores are spread by water splashes.
• Discoloration and translucency of the outer leaf tissues of Peperomia magnoliifolia, followed by the development of black streaking, is a suspected sign of iron deficiency.
• Cercospora leaf spot — causes tissue overgrowth or the appearance of yellow spots on the underside of leaves.
• Bacterial leaf spot (pathogen Xanthomonas campestris pv. dieffenbachiae) — manifests as water-soaked spots on the leaf blades and margins.
• Soft bacterial rot (pathogen Erwinia chrysanthemi) — causes wet rot of the foliage.
• The fungus Basidiophora humicola — triggers the development of water-soaked spots of a rusty or yellowish-green color.
• Ring spot virus — causes chlorosis, necrosis, and deformation, where young leaves curl into a cup-like shape.

In pilea, the main danger is root rot, caused by the pathogen Pythium sp. This disease leads to defoliation of plants and loss of commercial appearance. In addition, under conditions of excessive air humidity, tissue overgrowth is often observed on the underside of pilea leaves.

Protecting poinsettia, primula, and cissus from rot complexes

Poinsettia is considered a relatively resistant pot crop; however, if the roots are damaged by pests, it is easily affected by root rots. The pathogens of this complex are the fungi Pythium sp., Phytophthora sp., Thielaviopsis basicola, and Rhizoctonia solani. To control them, irrigation with fungicide suspensions is used. In addition, the secondary parasite Botrytis cinerea often settles on wilting parts of the poinsettia, and rot caused by Erwinia carotovora var. carotovora develops on damaged stems.

On young poinsettia plants, a light brown firm deposit sometimes appears, consisting mainly of calcium sulfate. Under this exudate, tissues can be damaged, which are then colonized by secondary microorganisms. The exact conditions leading to the formation of the deposit are currently poorly understood.

Primula is grown both in pots and in open soil, but it is susceptible to root rots everywhere. They are caused by species of Phytophthora and Thielaviopsis basicola. Phytophthora brown rot (Phytophthora primulae) is particularly dangerous, leading to wilting and death of plants. Its main diagnostic sign is the browning of the root vascular system. To protect primula, it is necessary to disinfect the soil and drench plantings with fungicide suspensions.

Ramulariosis (Ramularia primulae) is also common on primula, the conidia of which are carried by water. It manifests as round or irregular light brown spots with a chlorotic halo. Cissus suffers greatly from the fungus Pestalotiopsis sydowiana, which colonizes damaged or aging tissues and actively sporulates on them. Large black spots form on the leaves.

• Diameter of primula ramulariosis spots — up to 3 mm
• Diameter of Cissus spots — up to 1 cm
• Fungicide recommendations for poinsettia — pp. 352—353
• Fungicide recommendations for primula — p. 353