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This pathogen appears in early spring and has its maximum in June. The optimal conditions for the development of this mycelium are the presence of water and temperatures ranging from 18 to 24 °.
It is manifested by the formation of small yellow spots on the upper side of the leaf, while in the lower part we can see yellow powdery fragments. As the days go by these spots become darker and darker until they become blackish. This disease, if present in a massive way on a plant, causes a vegetative stasis and yellowing. Moreover, the rust almost never causes the immediate death of the plant, which perishes slowly, completing the vegetative cycle.
The main plants affected by this disease are: firs, clematis, geranium, rose, peach.
For the easy occurrence of the ideal conditions of the fungus the defense must be done in a preventive and repetitive way. The active ingredients most used in the struggle are fungicides such as copper and products containing zinc, such as zineb and ziram. Products to be sprayed in case of illness every 10-15 days and as a preventive every six months.
There rose rust it was described and identified for the first time in 1665 and is widespread throughout the world, particularly in North America and Europe. For this plant it is certainly not a pathology of primary importance except in specific pedoclimatic areas. In fact, the fungus needs specific temperatures and a particular degree of humidity due to spore germination, infection progression and survival.
It can become a frequent pathology in very cool areas and with high humidity, such as the alpine areas. The mushrooms at the base of the development of the rose rust they are nine and belong to the genus Phragmidium. Only then P.mucronatum and P.tuberculatum become habitual pathogens in cultivated roses (for example in the cut flower industry).
Some rose species are particularly prone to this disease: tea hybrids, climbing hybrids, perpetual hybrids and other types especially characterized by large and leathery leaflets. Even the subjects used as rootstocks can be sensitive: for example they are the Rosa Alba, the dog rose, the Laxa rose. The presence of an unsuitable holder can cause strong attacks and defoliation against the rose, especially in late summer.
Usually the first symptoms appear on the basal leaves and spread upwards until the climatic conditions remain optimal for its development.
If we live in an area with predisposed climatic characteristics we therefore pay particular attention to the rootstock chosen by the nurseryman. Eventually we can also focus on resistant plants reproduced simply by apical cuttings.
At the beginning of spring, small, powdery red-rust spots appear on the underside of the leaves. Within a few days, on the front of the leaf, you will notice yellow spots: gradually the spots widen and become even confluent. The amount of dust is increased and the color becomes more and more orange. On the front the leaf takes on a completely yellow and orange color and the shape becomes curved. The "fructification" of rust (aecia) measures about a millimeter in diameter and is sometimes surrounded by a pale yellow or reddish halo.
The fungus can also affect shoots, sepals and fruits. On the young stems the lesions appear long and narrow and can be the cause of a crooked, little vigorous growth and of the abortion of the buds.
Unfortunately, in spring the symptoms are often not very visible and therefore are ignored or underestimated.
The summer phase of the illness, however, is the most obvious and causes more serious damage both from the point of view of productivity and for the future survival of the subject.
Very small red-orange pustules form which contain the yellow-orange spores, both on the underside of the leaves and on the petioles. The summer stage is cyclical and can be repeated every 10-14 days with favorable weather. The leaves are continuously reinfected by the spores produced by the pustules. The severely affected leaves can become twisted and wither, falling 5-10 days after the first signs of infection. The young shoots grow deformed and take on a reddish color. These symptoms are followed by a general decline in plant vigor. In temperate climates, where there are never rigid temperatures (such as in the south or on the coast) this phase can last throughout the year. In the areas instead colder sets of black spores begin to form on the leaves and stems at the end of summer or early autumn and gradually take the place of orange ones. The general coloring of the leaf apparatus takes on colors ranging from dark brown to blackish. The black pustules can appear on the stems like incrustations.
The cycle of disease
In spring the black spores (more resistant, given that they have the ability to winter in black pustules, on dead leaves or on branches) germinate and produce other brown spores. These are carried by air currents and infect first the young leaves and jets of roses.
Development begins within the plant tissue thanks to several mycelia which produce infiltrating apparatus: with the naked eye they are highlighted as the very small circular red-orange spots on the back of the leaf, in spring. At this starting point, another stage of the mycelium develops and you get to have the large pustules towards the beginning of summer. Within these there is the maturation of other numerossisime spores. When they are ready, the pustule explodes, spreading the infection into the air. If in turn these spores reach other leaves and the climatic conditions are favorable, the disease spreads more and more. Penetration and infection occurs more easily on the back of the leaves. The new pustules appear within 10-14 days. At the end of summer there is the appearance of black spores resistant to cold and this completes the cycle. With mild winters the last phase can also be skipped and the production of summer spores continues.
The germination of all types of spores (spring, summer and wintering) occurs only if the surface of the plant remains continuously wet due to rain, fog or dew and the temperature is ideal. All types of spores can germinate in a range of temperatures ranging from 6 ° C to 27 ° C, but the ideal conditions are between 15 ° C and 21 ° C. Above 28 ° C (therefore almost everywhere in Italy during the summer), there is no germination, but the spores can survive up to a week.
This is why the problem becomes particularly serious only in mountain areas, where these temperatures are never reached. In almost all the rest of the peninsula the infection is blocked on the arrival of July and August. It may, however, once again become a major problem on the coasts and in the Center-South from autumn to spring.
Good agronomic practices are essential to eliminate the sources of infection.
- It is essential to try to introduce plants that are naturally resistant to this disease in the garden, in the garden and in the orchard. We buy only selected plants and from serious and specialized retailers. If we live in an area particularly subject to this problem (especially for climatic reasons) we also need to know in depth about which rootstocks are used and we buy only if resistant species are used or if reproduction occurs through apical cuttings.
- We carefully examine every new plant that we introduce into the garden. If we find traces of rust we eliminate all the affected parts and destroy them (possibly burning them, if allowed by the municipal regulation, otherwise bringing them to the green collection areas).
- We remove and destroy all the infected leaves that appear during the spring. We do not work or move the foliage when the weather is wet or when it is raining.
- carefully clean the entire rose garden before winter. We collect and destroy all the old leaves, even those still on the branches. We prune all the affected branches in winter or early spring, however before the new foliage appears. These practices, if followed precisely, should completely eliminate the wintering stage of the fungus.
- if we can clean wild or neglected roses around our home. They are often the first source of infection for ornamental plants.
- in greenhouses we always try to ventilate to prevent the humidity resulting from condensation from settling on the leaves. We avoid watering with rain, with water coming down from above, especially in the late afternoon or evening. We use drip irrigation tubes which, in addition to helping to save water, prevent these problems.
Rust prevention and cure products
If the suggested strategies fail to control rust, it may be necessary to distribute a fungicide every 7-10 days. It is important to remember that the entire aerial part must be covered by the product, especially the lower page of the leaves. Applications begin when the first buds appear and continue until July. You can then resume in September to finish with November.
The wettable products, to be distributed until dripping, are more effective than those in powder form because there is greater adherence and persistence. If the vintage is particularly rainy it may be necessary to proceed also every 4 days.
Before winter you can also vaporize the surrounding soil with the mixture in order to kill possible wintering spores.
The most effective active ingredients are: myclobutani, tebuconazole and triticonazole. For prevention and in organic farming wettable sulfur can be used.
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