Downy mildew is one of the most common diseases. The affected plants are vines, potatoes, lettuce, onions, cabbage, cucumbers, tomatoes, eggplants, ornamental plants, etc. It appears especially when the weather is rainy and cold.

Potato late blight (Phytophthora infestans)

Potato late blight, Phytophthora infestans is native to South America (Peru, Chile). It was introduced to Europe in the first half of the 19th century.

Symptoms. The potato late blight is manifested on the aerial organs and the tubers. The first symptoms are observed on the leaves in May-June, usually before the flowering of the crops. On the attacked leaves appear yellow spots, at first small, with a diffuse outline and more numerous at the edge of the leaflets. The spots gradually increase, turn brown, then brownish-black, and are surrounded by a yellow stripe, with irregular and diffuse edges.

On the underside of the leaf, at the periphery of the spots, there is a very fine, whitish fuzz, formed by sporangiophores and sporangia of the fungus. In relatively warm weather the down is very little distinct or no longer formed, and the spots stagnate in growth. In conditions of high humidity, the spots increase, being able to occupy the entire surface of the leaflets, which turn brown, dry, and darken.

On the stems, on the petioles of the leaves, and the peduncles, there are elongated, blackish-brown spots of variable sizes. If the spots enclose the stem all around, they dry completely, from the level of the spot to its tip, together with all the leaves. On the attacked tubers, the late blight manifests itself in the form of brown, brown-gray, or blue spots, near which the periderm of the tuber is slightly submerged.

The pathogen is Phytophthora infestans that live as a parasite in the mesophyll of the leaf near the spots. The transmission of the disease from one year to another is ensured by the tubers attacked by a late blight that has remained in the stock of the planting material. They can be a source of infection only in the case of monoculture when they give plants sprouted after harvest already infected. However, if there is a crop rotation or at least a 3-year rotation, this source of infection should not be considered. From the tubers attacked by late blight appear browned shoots at the place where they catch the tubers, and on the shoots appear the first leaves with spotted leaflets on which spores appear (generalized primary infection). Spores can reach the surface of the earth by rising water from the ground and then cause new infections (secondary infections).

The first infections are observed 35 days after planting if the rainfall is low. Or, even faster if the precipitation level is higher. The spores of the fungus spread through the wind, at least 11 km/day. This can lead to the destruction of 100 ha in the presence of an outbreak of infection, in areas with heavy rainfall and moderate temperatures.

Prevention and control. To prevent and control this disease, it is recommended to use healthy tubers when planting. Planting is done at 10-15 cm, to help reduce late infections in soil tubers. During the potato blight infection, sprinkle irrigation of crops will be avoided. They promote the rapid spread of the disease. Before storage, the potatoes destined for planting will be sorted, removing the infected, damaged, cut, or injured tubers.

During the growing season treatments with specific fungicides are performed.

Grapevine downy mildew (Plasmopara viticola)

The grapevine downy mildew, Plasmopara viticola is a disease native to North America, where it has been observed since 1834. Due to trade, the grapevine downy mildew was introduced to Europe. It first appeared in France, where the first infections were found in 1878. From this country it spread everywhere very quickly, causing considerable damage to vineyards.

GRAPEVINE DOWNY MILDEW – LEAVES ATTACK

The downy mildew attack on leaves looks varied depending on when the infection occurs. In spring, the spots are yellow, oily, and have a diffuse outline, reaching dimensions ranging from a few mm to a few cm (the stage of oily spots). Over time, the center of these spots turns brown, the leaves taking on a dry appearance (the stage of burns on the leaves). As the disease progresses, a white cottony growth can be seen on the lower leaf surface.

Depending on the climatic conditions (high temperature around 20-22 ° C and high humidity), which causes a rapid evolution, the oily phase no longer appears. The leaves no longer show that characteristic cottony fuzz on the underside.

GRAPEVINE DOWNY MILDEW – SHOOTS AND TENDRILS ATTACK

They have elongated brown spots. In wet weather, it is covered with a whitish fuzz, made up of the mass of mycelium with spores. On more evolved and lignified shoots, the downy mildew attack appears in the form of elongated brown spots. They appear near the nodes, near which the bark is dead.

Symptoms. The grapevine downy mildew attacks all green parts of the grapevine, especially the leaves.

GRAPEVINE DOWNY MILDEW – YOUNG CLUSTER ATTACK

It can be very dangerous in years with heavy rainfall. The infection takes place through the inflorescence tails, through the flowers, or the more developed part of the berries. The small, wet grape bunches turn yellow and are covered with mycelium and white spores. Or, it browns and dries in dry weather. The berries are covered with a whitish fuzz made up of mycelium and spores, because the young berries, not covered by the waxy layer, allow the mycelium to come out. The infection of the berries continues even after the grapes are larger and covered with the waxy layer. The attacked berries turn brown, shrivel up, and sometimes come off the bunch and fall easily.

The formation of mycelium and spores takes place in a relatively short time (6-10 hours) if the atmospheric humidity is high (95-100%) and the temperature is between 18-24 ° C.

The pathogen. In spring, the resistance spores of the fungus germinate on the surface of the soil soaked with water, at temperatures above 10 ° C. The maximum is 32 ° C and the optimum is 22-23 ° C. During germination, the growth cracks, and from the inside appears a mycelial filament with a large growth, which will fall to the ground. During the spring rains, the spores floating on the water around the stems are projected on the underside of the leaves, producing filaments of infection that penetrate the tissues of the host plant through the stomata. After the primary infections occur, the period of feeding the fungus on the leaf follows. During this phase, yellow spots of oily spots appear on the leaves.

The manifestation of the disease is marked by the appearance of mycelium (white fuzz) which is called spores. The spores are picked up by the wind and carried over long distances. They can maintain their viability for 7-8 days. Arriving on the organs of the vine, after being in the water drops, the spores produce filaments of infection that penetrate through the stomata (the natural openings of the leaf). Towards autumn, after the last secondary contamination, the fungus forms resistance spores, in the form of which the fungus overwinters.

Prevention and control. It is recommended to drain the lands that strongly retain water, deep plowing to bury the affected leaves that contain oospores, the control of weeds. Chemical treatments can be performed with specific fungicides.

Onion downy mildew (Peronospora destructor)

Onion downy mildew, Peronospora destructor is considered the most dangerous disease that can completely compromise the crops of chives, onions, or seed crops. The disease was discovered in 1841 in England. It has spread rapidly in all cultivating countries.

Symptoms. The disease manifests itself on leaves, flowering stems, flower stalks, and bulbs in all stages of development. On the leaves there are oval-elongated spots of light green color at first, then yellow. In wet weather, the spots are covered with a gray-violet fuzz. Over time, the tissues near the spots will necrotize and the leaves rot. In case of strong attacks, the mycelium of the fungus passes from the leaves to the bulb, being located in its apical part. Attacked bulbs do not grow, remain small, soften and rot early. In the section, through the affected bulbs you can see the gray color of the tunic. In warehouses with high temperatures and humidity, the infected bulbs are quickly destroyed.

On flowering stems, the disease is manifested by symptoms similar to those on the leaves, except that the spots are larger and more obvious. Frequently the flower stalks break near the spots at the slightest breeze.

The pathogen is Peronospora destructor. Its transmission from one year to another is done in two ways: through resistance spores that remain in plant debris and through infected bulbs. Primary infections in the spring are ensured by the germination of resistance spores. And, during the growing season, the spores that appear on the mycelium are the ones that spread the fungus. They appear at temperatures between 3-27°C, the germination of spores occurs in atmospheric humidity conditions for at least 10 hours. The spores are easily carried by the wind and will give new infections on onions, but also other plants such as garlic, leeks, shallot.

Prevention and control. It is recommended to apply crop rotation. Onion must return to the same field after 3-4 years. It is also recommended to plant material (bulbs, chives) and use seeds from healthy crops, harvest the bulbs in dry weather, and expose them to the sun for 8-10 days before storage.

During the growing season to control onion downy mildew treatments with specific fungicides are made.

Brassica downy mildew (Peronospora parasitica)

Brassica downy mildew, Peronospora parasitica is widespread in all countries with temperate climates, being very damaging, especially in the seedling phase of plants. The pathogen attacks both cultivated and spontaneous brassicas (crucifers).

Symptoms. The disease occurs more frequently on young plants in seedlings or immediately after being planted in the field. On the first leaves, there are spots of 1-3 mm of discoloration, yellow, evolving to gray-brown. On the underside of the leaves, towards the spots that have an irregular shape, a gray fuzz appears. Then, the spots enlarge, join, and if the weather is humid, large areas of the leaves are destroyed, which rot or dry out.
When the attack takes place later, even rotten bulbs can be reported. The attacked seedlings have gray or brown spots, the leaves are also spotted and contain infected seeds. The attack of this fungus on cauliflower prevents the formation of inflorescence, and a general infection can be seen on kohlrabis and radishes.

The pathogen is Peronospora parasitica. The fungus resists over the winter in the form of resistant spores or as a resistant mycelium in the roots of the seed crops. During the growing season, in humid weather, with frequent rains or fog, the fungus spreads very easily through the spores that appeared on the mycelium.

Prevention and control. It is recommended to gather and destroy the plant debris from in the field, disinfect or change the soil in the nursery, use healthy seeds. At the beginning of the first symptoms, to control brassica downy mildew, treatments with specific fungicides are recommended.

When the first symptoms appear, it is recommended to carry out treatments with specific fungicides.

Cucumber downy mildew (Pseudoperonospora cubensis)

Cucumber downy mildew, Pseudoperonospora cubensis is frequently found on cucumbers, melons, melons, squash, which is particularly harmful to crops in protected areas.

Symptoms. It attacks only the leaves, in the form of light green spots on the upper face, angular spots, well delimited by the veins of the leaves. The color of the spots changes to yellow, then to brown, and on the underside of leaves, a gray-purple fuzz appears. Under favorable conditions, the spots can join and a large part of the leaf is destroyed, and the plants will quickly remain without leaves. Infections occur in humid weather, at temperatures between 5-30°C (optimally 23°C) when spores appear that will germinate and give filaments of infection. They penetrate through the stomata of the leaves, which have at least half of the normal surface. The viability of the spores depends on the temperature, light intensity, and atmospheric humidity. In dry weather and at temperatures above 32°C the spores die. In dry tissues, spherical, yellow-brown resistance spores form.

The pathogen is Pseudoperonoospora cubensis. It is spread by rainwater or irrigation water. Sprinkler irrigation applied in the morning until 10 o’clock, when the spread of spores is at the highest level, starts a strong attack of this disease. The transmission of the fungus from one year to another is ensured by the resistant spores present in the plant debris, but the infections from the following years can also come from the spores brought by the wind from the areas with mild winters, where they remain viable on dry twigs.

Prevention and control. As the first measure to prevent cucumber downy mildew, it is necessary to burn the debris of the infected plants or to bury them in depth. In the vegetable crop, the pumpkins will return after at least 4 years on the same soil. In protected areas, good ventilation of the crop will be ensured, and in case of attack, the humidity will be reduced.

During the growing season, treatments with specific fungicides are recommended.

Lettuce downy mildew (Bremia lactucae)

Symptoms. This disease occurs on the basal leaves of lettuce, in different phases of growth. It appears as large yellow spots. These spots can enlarge turn yellow and can destroy the whole leaf. If the attack is installed on the first 2 leaves, the seedlings perish. On the leaves of developed plants, the spots enlarge between the veins they can be 3-15 mm in diameter or even more. Near the spots, the underside of the leaf appears a fine, whitish fuzz made up of the mycelium with spores of the fungus.

The pathogen is Bremia lactucae, a pathogen that evolves in optimal conditions. Specifically, if the temperature is maintained between 15-17°C and there is high humidity. In 3-8 hours from the arrival of the spores on the leaves, they give germination filaments (if the temperature is higher than 12°C). The spores move in the water on the leaves and give infections, and after 6-8 days a new generation of spores appears. Their formation is prevented by temperatures higher than 20°C and drought. The transmission of the fungus from plant to plant is done through spores carried by wind and rainwater and irrigation. From one year to the next, the fungus resists through resistance spores that will germinate next spring and will form infection spores again. In addition to salad, this pathogen can attack endives and artichokes.

Prevention and control. As preventive measures for lettuce downy mildew, it is recommended to gather and destroy the attacked plants. Lettuce cultivation will be established only in lands well cleaned of plant debris, in a 4-year crop without endives or artichokes, and will not be irrigated by sprinkling.
Chemical treatments are recommended to be applied only to signal the attack, using specific fungicides.

Tomato late blight (Phytophthora infestans)

Tomato late blight, Phytophthora infestans was reported in Europe in 1847. It appears on tomatoes, especially in the second cycle of culture. In greenhouses and polytunnels, the crop damages can vary from 30% to the entire compromise of the crop.

At first, the spots are greenish-yellow, then gradually turn brown. In the case of intense attacks, the disease also manifests itself on the stems, in the form of small, brown spots, which surround the stem on large portions. Usually, the stem rots, bends, or breaks.

Symptoms. Tomatoes can be infected in all growth stages, the leaves showing on the edges or towards the top spots with a damp appearance (dark green). Affected tissues wither, turn brown and the leaves curl up to the topside. In conditions of atmospheric humidity on the lower face of the leaves, near the spots appears a fuzz, not too dense, white. The peduncle of leaves, the shoots, and the attacked stems, have elongated brown spots, superficial, without whitish fuzz. The attacked inflorescences show on the leaves and at the base of the flowers brown areas.

The significant attack is on the fruits, which can be attacked in all stages of development. The pathogen produces large brown-olive spots and quickly destroys the whole fruit. Tomatoes become hard, rough, and ripe ones have light brown spots, on the surface of which appear concentric areas. On fallen leaves where there is high humidity, the fungus forms a mycelium with many spores. In autumn crops in greenhouses (second cycle), the attack evolves rapidly, due to variations in day-night temperature, which cause condensation of water on the vegetative organs, condensation that eases infections.

Tomato’s late blight pathogen is Phytophthora infestans. It occurs especially in years with heavy rainfall and moderate temperatures.

Prevention and control. It is recommended to gather and destroy plant debris, seedlings to be produced from healthy seeds, avoid growing tomatoes in the vicinity of potatoes or on the same land, where potatoes were grown in previous years. Chemical treatments are applied at intervals of 6-7 days with specific fungicides.