This disease, present especially in Europe, because of its wide diffusion and seriousness, is considered the most important Gerbera pathology, especially in soilless cultivation. It infact in the cases of severe infestation causes the death of the plants in the space of 10-15 days.
Potentially the disease can occur in any period of the year, nevertheless, it spreads much more quickly when there are high levels of humidity in the greenhouses and in the growing media the temperatures are in the range of 22-25°C. The fungus prefers cool environments; with values higher than 25°C it gradually becomes harmless.
Its diffusion takes place very quickly through the zoospores, unicellular organisms, provided with flagella (small appendixes) that allow the pathogen to swim in a film of water to reach the roots and the crown. When conditions are wet and free water abounds, Phytophtora will produce zoosporangia that produce zoospores which infect roots and again produce more zoosporangia.
This repeating cycle can increase the number of spores rapidly in soil so that epidemics and rapid loss of plants is very possible. When conditions are unfavorable for zoospore production (drying or lack of host roots) the fungus makes resting spores (Oospores and Chlamydospores) which will germinate when the time is right.
The phenomenon flourishes when irrigation is carried out by means of large amounts of water being applied by hosepipe and then being allowed to spread of its own accord. Soilless cultivation, in bag or containers systems in which more plants are into contact with eachother by means of the root apparatus, also favours the spread of this phatogens.
In soilless cultivation using a closed system (recirculation of the nutrient solution) the risk of possible infestation and diffusion of a high rate of the pathogen is much higher, so that (as already mentioned in previous chapters), it is necessary to have very efficient systems of sterilization of the solution.
• Presence of tender plant tissue due to the age (young plants) or to Nitrogen excess;
• The repeated use of the same soil for cultivating the same plant (monoculture), that favours more resistant and aggressive organisms;
• Contamination by infected residues from previous crops, or from the reuse of substrates not properly sterilized ;
• The use of contaminated irrigation water. The surface waters of rivers and torrents are generally rich in pathogenic fungi, whereas the use of deep underground waters avoids the problem;
• The survival of the fungus in significant amounts in the soil and in vegetable residues in resting spores (Oospores, Chlamydospores);
• Transportation of the spores by air;
• The transmission through contaminated machinery or hands of the operator;
• The use of manure or peat. Research shows that the peat can contain resting spores of Phytium, and even more of Fusarium;
• Some insects can spread the pathogenic fungi among the plants of Gerbera;
• Unsuitable irrigated systems (e.g. sprinkling system below and above the foliage);
• High soluble salt (E.C.) that often lead to Phytium root rot probably because of root injuries.
Some research has also shown that the pathology becomes more serious when the plant suffers from attacks of Nematodes (genus Meloidogyne), probably because of the lower resistance to this in the points in which the root galls are formed. The symptoms of Phytophtora are shown by a sudden collapse of the whole plant foliage (it appears “open”) due to the loss of turgidity of the petioles that in a short time collapse onto the soil, leaving the middle “empty”. The foliage and the stems are of a colour which varies from dark to light green. The base part and the plant crown turn brown, with a gradual and a complete collapse of the whole leaf and flower apparatus. The leaves collapse completely onto the soil (“open plant”) and subsequently die. Moreover, the roots turn dark. When the crown is cut diagonally we can see it is completely dark. The fungus enters the plant at the crown and the collar; the incubation phase can last up to several weeks (it depends on the thermo-hygrometric conditions in the greenhose).
The disease, due to the symptoms that it produces, can be confused with other pathogens. Isolation and identification of the pathogen is the only way to accurately identify the cause of crown rot.
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