Flower watering methods for different seasons and the occurrence patterns of rust disease in seedlings

Rust disease is one of the more common and serious types of diseases affecting flowers and landscape greening trees. It is distributed worldwide and occurs in many parts of China. There are many types of rust disease, and in landscaping it mainly harms nearly a hundred species of flowers and trees in the Rosaceae, Fabaceae, Liliaceae, Poaceae, Pinaceae, Cupressaceae, and Salicaceae families. After infection, transpiration and respiration accelerate, photosynthesis weakens, growth vigor declines, leaves yellow and fall early, or branch tumors and fruit deformities are caused, reducing yield and ornamental value, and in severe cases even killing the seedlings, causing major losses.

1. Symptom characteristics

The disease is named for the large number of rust-colored spore masses appearing on diseased plants. On leaves, stems, and other parts, light green small spots first appear, then expand into rust-brown blister spots. After the epidermis ruptures, yellow-brown powdery material is released, which is the uredinium; some form orange-red to black small powdery masses, which are the telium; some produce honey-yellow to dark brown dots or granules, which are the spermogonium; and some produce many light yellow, gray-yellow to gray-brown slightly raised or bristle-like structures, which are the aecium. On branches and stems, tumors, witches' brooms, and rough bark appear, and fruits become deformed or split open.

2. Pathogen and its life history

2.1 Pathogen types

Rust disease is caused by parasitic rust fungi. Rust fungi belong to the kingdom Fungi, subphylum Basidiomycotina, order Uredinales. The common rust diseases infecting flowers and trees include the following.

2.1.1 Puccinia genus (Puccinia)

For example, it causes daylily rust, turfgrass stem rust, leaf rust, stripe rust and crown rust, zoysia rust, iris rust, chrysanthemum cypress rust and black rust, aster rust, sunflower rust, geranium rust and hollyhock rust, more than 10 kinds in total.

2.1.2 Single-spore rust genus (Uromyces)

For example, it causes gladiolus rust, carnation rust and aster uromyces rust, etc.

2.1.3 Multi-spore rust genus (Phragmidium)

There are rust diseases of rose, multiflora rose and Chinese rose, etc.

2.1.4 Gymnosporangium genus (Gymnosporangium)

There are more than 20 kinds of rust diseases of Malus halliana, Chaenomeles speciosa, Malus genus, Pyrus genus, etc., and rust diseases of various cypresses such as Sabina chinensis and Sabina chinensis 'Kaizuca'.

2.1.5 Cronartium genus (Cronartium)

There are peony and Chinese peony rust, pine gall rust, Korean pine blister rust, Mongolian Scotch pine blister rust, etc.

2.1.6 Coleosporium genus (Coleosporium)

There are aster rust, Korean pine needle rust, Chinese pine needle rust and Mongolian Scotch pine needle rust, etc.

2.1.7 Phakopsora genus (Phakopsora)

There is chrysanthemum brown rust, etc.

2.1.8 Melampsora genus (Melampsora)

There are Chinese white poplar rust, larch-poplar rust, Euphrates poplar rust, etc.

2.2 Life history

2.2.1 Spore types

The simplest life history of rust fungi has only teliospores (represented by symbol III), while the most complex has five types of spores, namely spermatia (○), aeciospores (I), urediniospores (II), teliospores and basidiospores (IV).

2.2.2 Life history types

Typical rust fungi must sequentially go through 5 different developmental stages and produce 5 types of spores to complete the entire developmental process, while some rust fungi have variations, lacking one or more of these spore types.

The long (cyclic) life history type, which produces 5 spore stages, is called the long life history type or long-cycle life history type. The long life history type can be further divided into two types: the heteroecious long life history type and the autoecious long life history type. In the former, the host infected after aeciospore germination is different from the original host plant producing the aecium, and their genetic relationship is relatively distant, such as carnation rust, whose ○ and I stages parasitize plants of the Euphorbia genus, and whose II and III stages parasitize plants of the Dianthus genus. For Cronartium flaccidum, its ○ and I cause pine blister rust on masson pine, Chinese pine and other pine plants, and the perennial mycelium in the diseased part can continue to produce aeciospores the following year, while its II, III and IV stages are on Chinese peony and peony. For Cronartium quercuum, its ○ and I are on pine trees, and II, III and IV are on Quercus of the Fagaceae family. In the autoecious long life history type, all spore stages are completed on the same host, such as rose rust caused by three Phragmidium species, sunflower rust caused by Puccinia helianthi, and broad bean and pea rust caused by Uromyces fabae, etc.

The life history type lacking urediniospores is further divided into heteroecious and autoecious parasitism. In heteroecious parasitism, such as rust of Malus halliana, pear, apple, etc. caused by Gymnosporangium asiaticum and Gymnosporangium yamadae, ○ and I occur on Malus halliana, etc., and III and IV occur on cypresses such as Sabina chinensis, Sabina chinensis 'Kaizuca', Sabina chinensis and Juniperus rigida, producing tumors of varying sizes on needles and small branches, called gall or telial horns, which swell upon absorbing water into orange-yellow gelatinous masses.

In addition, there is the short (cyclic) life history type, with only ○ and I stages, overwintering as mycelium in diseased parts and infecting young buds the following spring, such as plum blossom rust. There is also the type with only III and IV stages, such as chrysanthemum white rust and mallow rust, etc. These rust fungi are all autoecious parasites.

3. Disease epidemic patterns

3.1 Infection cycle

The infection cycle of rust fungi is relatively complex, and different rust fungi differ greatly. The pathogen overwinters, some as mycelium or mycelium and urediniospores within diseased tissues of plant buds, branches and leaves, seed-bearing mother plants, etc., and the mycelium can survive for many years; some as teliospores and/or urediniospores on diseased residues such as dead branches and fallen leaves; some can overwinter both in diseased residues and within diseased tissues. Teliospores germinate to produce basidiospores, basidiospores infect to produce spermatia and aeciospores, and aeciospores infect to produce urediniospores and teliospores. Among the five spore types, only aeciospores, urediniospores and basidiospores can infect the host. Urediniospores can repeatedly cause secondary infection, and for the autoecious rose rust, both its urediniospores and aeciospores can cause secondary infection. Gymnosporangium rust lacks urediniospores, so it has no secondary infection and only one primary infection. Spores are mainly spread by wind, and also by raindrop splash. After germination, aeciospores and urediniospores generally invade the host through stomata, while basidiospores can directly penetrate the host epidermis or invade through stomata. Generally, young buds, tender leaves and tender branches are susceptible to infection and disease. For heteroecious rust fungi, the infection cycle can only be completed and disease caused when the alternate host plant is present.

Among rust pathogens, only very few species such as safflower rust are transmitted through seeds, while most are transmitted through diseased seedlings, cuttings and scions, etc. Spores spread over a relatively wide range with air currents; the basidiospores of Gymnosporangium asiaticum can spread to about 5 kilometers away. Some rust fungi spread their urediniospores over long distances with high-altitude air currents of more than a thousand meters to different natural geographical regions, different countries, and even different continents hundreds to thousands of kilometers away, spanning dozens of latitudes.

3.2 Disease epidemic factors

3.2.1 Climatic factors

Among various climatic factors, air temperature, air humidity and precipitation have a greater impact on disease occurrence. For most rust fungi, growth and development, spore production, germination and disease occurrence are favored by slightly lower warm weather. Generally, disease is more severe when the temperature is 10-26°C. Too high or too low temperatures inhibit spore formation, survival, germination and infection, so in most parts of China, spring and autumn are the periods of frequent rust disease. The disease is closely related to air humidity; when the relative air humidity remains above 80% for several consecutive days, especially at saturation humidity, the disease becomes severe. Spores can only germinate in water droplets or water films, so rainy weather, especially many rainy days, heavy dew or foggy weather, causes disease epidemics.

3.2.2 Maintenance and management factors

Landscape maintenance and management, that is, improving microclimatic conditions, is beneficial to the growth of landscape plants and can also enhance plant disease resistance. Flowers and trees planted in deep, loose, fertile loam rich in humus can reduce disease occurrence, while soil that is compacted and clayey, or poor and sandy soil with poor water and fertilizer retention, leads to more severe disease. For example, if planting is too dense, sunlight is insufficient, nitrogen fertilizer is applied excessively, flowers and trees grow excessively, the nursery is waterlogged for a long time, plants grow poorly, and disease resistance is weak; if management is extensive, timely pruning is not carried out, garden sanitation is poor, there are many diseased branches and fallen leaves, and a large amount of pathogen exists, the disease will be severe.

4. Integrated control

4.1 Eliminate and reduce sources of inoculum

To control heteroecious rust diseases, such as when configuring landscape plants in newly built parks, ornamental plants and alternate host plants should be strictly separated. For example, rust of Malus halliana, apple, pear, etc. should be separated from the alternate host cypress by 5 kilometers; rhododendron and spruce, hemlock, and aster and two-needle pine, three-needle pine, etc. must not be mixed planted. If they have already been mixed planted, it is best to thoroughly remove the alternate host; if it is difficult to remove, the control of diseases on the alternate host should be strengthened. Apply fungicide when spores are about to disperse for prevention. To reduce spore entry and disease transmission, plant the plants to be protected upwind of the disease-transmitting plants to reduce disease occurrence. To control autoecious rust diseases, from late autumn to early spring of the following year or during the plant dormancy period, thoroughly clean up fallen leaves, fallen fruits and dead branches and other plant residues where diseases and pests lurk in the garden, and frequently remove diseased branches and leaves during the growing season and dispose of them centrally, which can reduce the source of inoculum.

4.2 Strengthen maintenance and management

Improve the plant growth environment and enhance disease resistance. Before establishing a garden, choose suitable sites, improve the soil, increase soil permeability, improve soil fertility, and arrange the garden irrigation and drainage system well; select robust, disease- and pest-free branches as cuttings, scions and other vegetative propagation materials, and strictly remove pathogens; control planting density, which should not be too dense; remove standing water in time; fertilize scientifically, applying more decomposed organic fertilizer and phosphorus-potassium fertilizer, and not applying nitrogen fertilizer excessively; frequently prune and trim branches, remove diseased, pest-infested and weak branches, so that ventilation and light transmission in the garden are good; protected cultivation should strengthen ventilation and air exchange to reduce humidity in the greenhouse.

4.3 Chemical control

4.3.1 Winter application

From late autumn to before bud break the following year, after cleaning the garden and pruning diseased branches, apply fungicide for prevention. You can spray 2-5 degrees lime sulfur mixture, or 45% crystalline lime sulfur mixture at 100-150 times dilution, or sodium pentachlorophenate at 200-300 times dilution, or a mixed solution of sodium pentachlorophenate and lime sulfur mixture. When preparing, first dilute sodium pentachlorophenate with 200-300 times water, then slowly pour it into the lime sulfur mixture liquid, stirring thoroughly while pouring, to adjust to a 2-3 degrees Baume solution. Do not add sodium pentachlorophenate powder to the lime sulfur mixture without adding water first, to avoid precipitation. To control rust on the alternate host cypress, spray 1-2 times in early spring in the first and middle ten days of March to kill the overwintering teliospores.

4.3.2 Growing season application

At the early stage of disease in flowers and trees, spray 0.2-0.3 degrees Baume lime sulfur mixture, 45% crystalline lime sulfur mixture at 300-500 times dilution, or 70% mancozeb wettable powder at 500 times dilution, or 62.25% Xiansheng at 600 times dilution, or 500 times 80% Dacotech M-45, or 70% thiophanate-methyl at 1000 times dilution, or 25% triadimefon at 1500 times dilution. Newer fungicides for controlling rust also include 12.5% diniconazole at 3000-4000 times dilution, or 43% Tilt at 4000-5000 times dilution, 25% Folicur at 1000-1500 times dilution, 25% Tilt at 1000-4000 times dilution, 25% mepronil at 1000 times dilution, 30% Amistar at 3000 times dilution, 50% Cuibei at 3000-5000 times dilution, 10% Shigao at 3000-5000 times dilution, 25% Fuxing at 5000-8000 times dilution, 50% Leinengling at 1000-2000 times dilution.

4.4 Strict quarantine

Many tree branch and stem rust diseases are quarantine objects. Seedlings should be introduced from disease-free areas, and vegetative propagation materials such as cuttings should be collected from disease-free mother plants.

4.5 Breeding disease-resistant varieties

There are obvious differences in rust resistance among flower and tree species and varieties. Therefore, breeding rust-resistant flower and tree varieties is an economical and effective way to control rust disease.