Diagnostics of marginal leaf scorch in freesia and specific agricultural practices for greenhouse roses

Freesia marginal scorch diagnostics and rose agrotechnics

Marginal scorch manifests as necrosis on the tips and edges of aging freesia leaves. This symptom points to any damage to the root system or exposure to phytotoxic chemicals. For example, scorch often occurs when watering with water containing a high concentration of fluoride. There are no absolutely resistant freesia cultivars, but the Tozsa cultivar is particularly sensitive to this problem. Scorch symptoms are more pronounced in plants grown from seed, as well as under intensive lighting and sharp fluctuations in the water regime.

Greenhouse roses are propagated by budding or grafting. Budding is carried out in the field, after which 12–18-month-old bushes are transplanted into the greenhouse. Grafting is done with a dormant bud, and then the grafted bushes are grown in warm hotbeds until complete union. Most roses are propagated on Rosa canina or Rosa canina inermis rootstocks, while Rosa manettii and Rosa indica major species are used more often for producing 12–18-month-old plants.

• Planting density — 7–8 bushes/m² (69–79 thousand/ha)
• Layout on 1.2 m wide beds — 25–30 х 25 cm or 25–30 х 12.5 cm
• Winter dormancy period for roses — 6–8 weeks (December — January)
• CO₂ top dressing in the greenhouse — up to 0.1 g/l (October — April)
• Plantation service life — 6 years

For most rose cultivars, a winter dormancy period is traditionally provided, during which pruning of the bushes is carried out. However, some modern cultivars are grown without a dormancy period: flower harvesting is conducted year-round, and pruning to reduce bush height is performed in winter or summer. Temperature regime in the greenhouse depends directly on the season, and the productivity of the plantings is determined by the characteristics of the cultivar.

Powdery mildew control in greenhouse conditions

Powdery mildew, caused by the fungus Sphaerotheca pannosa (S. pannosa), affects exclusively Rosae species. A white powdery coating forms on the leaves, stems, and buds of diseased plants, which causes the produce to completely lose its marketability. The disease begins on the youngest leaves with the appearance of reddish-purple spotting, causing them to curl. Fungal conidia are carried not only by the wind but also on the clothing of greenhouse workers. The pathogen overwinters as mycelium in dormant buds and as pustules on stems, and in the spring, it progresses rapidly as temperature and air humidity increase.

All modern rose cultivars are susceptible to powdery mildew to varying degrees. Plant resistance is strongly influenced by the type of rootstock: rootstocks with a vigorous, actively growing root system provide roses with better protection against the disease. To prevent the development of an epiphytotic, it is necessary to regularly carry out a complex of sanitary and protective measures in the greenhouse.

1. Control the microclimate in the greenhouse, striving to maintain low relative air humidity.
2. During the winter dormancy period, perform deep pruning of the plants, removing the maximum number of affected shoots.
3. Carefully collect and remove all plant residues from the greenhouse, preventing their accumulation.
4. During the dormancy period, carry out an eradication treatment with fungicides on both the plants themselves and the greenhouse structures.
5. Throughout the growing season, regularly treat the plantings with fungicides according to schedule.

A traditional and effective method of protecting roses is fumigation with sulfur vapors using electric heaters (sulfurizers), which are placed 10 meters apart from each other. Under normal conditions, fumigation is carried out during daylight hours. If conditions favorable for the development of powdery mildew occur in the greenhouse, sulfurizers are turned on for 6 hours every night.

Currently, instead of sulfur, high-volume spraying with fungicides is used much more often. Dinocap, dodemorph, bupirimate, imazalil, chlorothalonil, drazoxolon, and nitrotal-isopropyl, as well as sulfur-based preparations, possess efficacy against powdery mildew. Alternating fungicides reduces the risk of pathogen resistance development.

Downy mildew (Peronospora sparsa, syn. Pseudoperonospora sparsa)

Downy mildew occurs rarely, but can be highly destructive. Its initial symptoms are easily confused with signs of powdery mildew infection. Small reddish spots appear on the youngest leaves, and the leaf gradually deforms. On aging leaves, the symptoms are very typical: initially, light green, somewhat diffuse zones on the leaf blade surface, which lighten and then turn gray or brown. At this stage of the disease, leaf drop begins — usually the first sign noticed by the grower. Leaflets with very mild symptoms fall off at the first touch to the plant. On aging tissues, downy mildew can be confused with chemical burns or physical damage. Accurate diagnosis is complicated by the fact that the pathogen is not always easy to detect. Sporangiophores (conidiophores) may develop on the underside of leaves, but their detection requires examination under a magnifying glass.

Fig. 15.2. Symptoms of downy mildew on the rose leaf surface. Initially, the affected areas of light green color are clearly delimited by veins.

Fig. 15.3. Downy mildew at a later stage of development; signs of burns and some deformation appear on the leaves. Sporangia (conidia) easily detach from the sporangiophores, especially under conditions of decreasing humidity, when the sporangiophores twist and shake off the sporangia. Airborne sporangia germinate in droplet moisture and, after infecting the host, produce a new generation of spores; at a temperature of about 15 °C, this process is completed in 10-14 days. Oospores also develop in the tissues of infected leaves, but their role in the development of the disease has not been established.

Peronospora sparsa affects only plants of the Rosaceae family. Commercial cultivars do not possess resistance to the disease, and some are particularly susceptible to it. During prolonged periods of high relative humidity and surface moisture, epiphytotics develop. Most often, this occurs in autumn or spring.

To protect roses from downy mildew, it is necessary to avoid high humidity and surface moisture in the greenhouse. This is especially important when using thermal screens for heat conservation and when enriching the atmosphere with CO₂, as both processes are associated with a reduction in the number of ventilations and, consequently, an increase in relative humidity.

High-volume spraying with fungicides is accompanied by a weakening of disease spread and protects against epiphytotics. The following preparations are most effective against this disease:

• copper-based preparations;
• zineb;
• mancozeb;
• fosetyl-aluminum (Aliette);
• mixtures of metalaxyl and mancozeb.

As long as environmental conditions are favorable for disease development, spraying must be carried out regularly.

Grey mold — is a common disease of the rose, but it does not always pose a danger. During propagation or shortly after planting, young plants are sometimes severely affected by it. On young shoots, the pathogen causes browning, and if the affected tissue girdles the shoot, it wilts and dies. Under moist conditions, fungal sporulation occurs on the affected tissue, appearing as typical symptoms of grey mold. At very high relative humidity, flowers are also affected, especially those of light colors, on which small purple-red spots are visible. The pathogen of grey mold grows equally well on dead tissues and on any other plant debris on the soil surface.

Control. When controlling environmental factors (preventing prolonged periods of high humidity), grey mold does not pose a problem. High-volume spraying with iprodione or vinclozolin is also effective against the pathogen.

Due to the widespread use of sulfur fumigation, black spot was not previously encountered in greenhouses, but it now appears occasionally, especially in new rose plantings. Typical black, round spots up to 1.5 cm in diameter develop on the young leaves of affected plants, and the diseased leaves fall off. Infection apparently occurs during propagation by budding in open soil before the roses are transplanted into the greenhouse. On the affected leaf areas, the pathogen forms numerous conidia, which are easily spread from diseased plants to healthy ones by water droplets, infecting leaves and stems.

Control. All affected tissues must be removed from the plants and from the soil surface. To prevent the spread of the pathogen, water splashing should be avoided. High-volume spraying with the following preparations provides effective protection against black spot:

• benzimidazoles;
• chlorothalonil;
• captan;
• mancozeb.

Treatments are carried out regularly immediately after transplanting the plants.

This disease is atypical for greenhouse roses, but it can be very harmful if it develops. The pathogen most often affects leaves, and less frequently stems and flowers. Pustules up to 5 mm in diameter form on the underside of the leaves; due to their bright orange color, they serve as an accurate diagnostic sign. Round yellow spots are visible on the surface of the affected leaves. Similar but larger pustules can appear on the stems, causing their deformation and sometimes death. This is the aecial stage of the pathogen. Infection by aeciospores is accompanied by the formation of small, less bright, but more numerous uredopustules. By the end of summer, black teliospores form instead of uredopustules, and the similar size... Fig. 15.5. Black spot on rose leaves (pathogen Diplocarpon rosae). Affected leaves often fall off. Fig. 15.6. Symptoms of rust are visible on both sides of rose leaves. On the upper side, these are round yellow spots showing the location of orange aecia on the underside of the leaf blade.

Fig. 15.7. Later, reddish-brown uredopustules (a) and black teliospores (b) of the rust pathogen appear on the underside of the leaves. Teliospores are resistant to unfavorable environmental factors and germinate, forming basidiospores and releasing them into the air. Landing on the tissues of a suitable host plant, basidiospores germinate and infect it, i.e., the infection cycle is repeated, starting from the aecial stage. Teliospores are resistant to adverse environmental conditions and can survive in the absence of hosts, i.e., between crops. The hosts of the rust pathogen include only members of the genus Rosa, and the dog rose Rosa canina is often affected and serves as an important source of infection.

...spread mainly by air and water droplets. Germination occurs at high relative humidity or in free water.

Control. Under conditions unfavorable for development, rust is never harmful. Affected tissues should be removed, and with the appearance of signs of the disease, high-volume spraying with benodanil, oxycarboxin, or mancozeb should be carried out regularly.

Canker (Coniothyrium wernsdorffiae, syn. Coniothyrium fuckelii)

This pathogen appears quite rarely, in particular on poorly developing bushes or if it initially colonizes dead tissues and penetrates healthy ones through them. Light brown spots, often with a gray center, appear on the affected stems. If the lesion circles the stem, signs of wilting appear. Pycnidia form on dead tissues, and spores are spread by water splashes.

Control. All dead tissues must be removed to prevent colonization by the pathogen. Whenever possible, the affected stem areas are cut out. Spraying with mancozeb suppresses the development of canker. Severe outbreaks of the disease indicate errors in agricultural practice. In this case, the entire cultivation technology for roses should be carefully reviewed to identify possible causes of poor plant development and the corresponding appearance of the disease.

Verticillium wilt is relatively uncommon, but sometimes appears in greenhouses. Affected plants are characterized by stunting; their aging leaves become chlorotic, and gradual defoliation begins. Unlike many other wilts, vascular browning in roses is often unnoticeable. Affected plants rarely die; sometimes signs of recovery appear in late summer and early autumn.

The pathogen is soil-borne, but it can infect plants during grafting or budding. It has a wide host range, affecting many greenhouse crops, including tomato, cucumber, pepper, and chrysanthemum.

Control. Affected plants must be removed, and soil disinfection must be carried out before planting a new crop. Effective chemical control methods for Verticillium wilt of rose have not been developed, although irrigation with benzimidazoles suppresses this disease well in herbaceous crops.

Armillaria root rot, honey fungus (Armillaria mellea)

On roses, root rot, caused by Armillaria mellea, sometimes appears in greenhouses built on cleared sites of forest belts or other forest plantations. Affected plants grow poorly, but clear symptoms of the disease are difficult to detect. Sometimes ulceration develops; to determine the cause of the disease, it is necessary to examine the root system of diseased bushes. If, upon careful removal of the bark and inspection of the underlying tissues, white fan-shaped mycelium of the fungus is visible, the pathogen is honey fungus. Diseased tissues emit a sharp mushroom smell. A. mellea forms rhizomorphs, which grow along the roots and even rise to the soil surface. In this way, the pathogen spreads and infects neighboring plants. In the soil, honey fungus survives for a long time in the presence of food sources, such as rotten stumps or plant residues from dead trees.

Control of honey fungus and bacterial crown gall of rose

The appearance of honey fungus on roses requires quick and radical cleaning of the site. This fungus actively feeds on woody residues, so half-measures will not work here. To completely eliminate the food source for the pathogen, it will be necessary to carry out deep cleaning of the plot.

1. Remove affected rose bushes.
2. Thoroughly dig up the soil to completely eliminate old stumps and roots.
3. If necessary, remove a layer of soil to a depth of one meter or more.
4. Carry out mandatory soil disinfection before replanting roses.

Bacterial crown gall of roses becomes dangerous when growing a crop in one area for many years. Under such conditions, a critical volume of plant residues accumulates in the soil, in which the soil-borne pathogen persists. The disease causes secondary proliferation of rootstock or cultivar tissues at the very soil surface or slightly below it. As a result, large galls 10–20 cm in diameter are formed, and affected bushes begin to lag significantly in growth.