Crops
Pythium spp. (Fontaine des Semis)
The Pythium are best known for tackling early tomato in nurseries or after sowing in the field, affecting the seeds or young seedlings before or after germination (damping off, damping-off ). They are sometimes rife on adult plants under special conditions of soil culture, hot and very humid, encountered in tropical areas. The development of soilless crops on substrates and NFT has allowed these chromists to settle in these production systems and cause root alterations ( Pythium root rot ) sometimes spectacular throughout the production cycle of this plant.
Their symptoms are mainly observed on the organs present in the soil or the substrate (roots, crown), but they can also develop on the foliage or the fruits of the tomato.
In nurseries, Pythiums are able to inhibit seed germination or disrupt the development of young seedlings after rapidly colonizing the roots and / or crowns (Figures 1 and 2). Affected tissues, often succulent, tender and therefore very sensitive, are moist and quickly turn brown, rot and die. Some roots, but especially the portion of the stem located on the surface of the soil or substrate, may present a constriction (figure 2). Subsequently, it is quite common to observe foci of seedlings that wilt, turn yellow, wither and / or collapse and more or less disappear (Figures 3).
It should be noted that aerial attacks on seedlings are possible during particularly humid and rainy climatic conditions. Thus, wet and brown lesions can appear on the leaves, petioles and stem and lead to their death. Such symptoms have been reported especially in Florida and associated with P. myriotylum .
The effects of Pythium spp. can persist beyond the nursery, on plants in the development phase after planting and adults. Symptoms resulting from the direct effect of Pythium spp. on the tomato of those linked to their consequences on the proper functioning of plants. The first mainly concern the roots and the crown. In many situations, yellowing , more or less marked and generalized root browning, the disappearance of numerous rootlets are observed. (Figures 4 and 5) Moist and dark lesions may develop at the neck and extend over a few centimeters * ( P. aphanidermatum , P. myriotylum ).
These symptoms can have various consequences: the appearance of symptoms of apical necrosis on the fruits, a reduction in plant growth and sometimes yields, more or less rapid and reversible wilting, drying out, plant death. They are mainly observed in soilless crops, sometimes in soil but in tropical production areas.
Note that a synergy of damage was observed between P. aphanidermatum and the root-knot nematode Meloidogyne incognita .
Especially green and ripe fruits, in contact with or near the ground, can be affected by certain species of Pythium spp. These cause wet lesions which quickly develop into a rot which is the cause of liquefaction and collapse of the fruits. A white cottony mycelium is sometimes visible on the lesions.
Only the observation of oospores (sometimes other structures such as sporangia or chlamydospores) on and in the affected tissues or the realization of isolations on artificial media, make it possible to diagnose with certainty the intervention of these fungi (figures 6 and 7).
For additional information on these Oomycetes, you can consult the sheet Miscellaneous Oomycetes .
Their symptoms are mainly observed on the organs present in the soil or the substrate (roots, crown), but they can also develop on the foliage or the fruits of the tomato.
In nurseries, Pythiums are able to inhibit seed germination or disrupt the development of young seedlings after rapidly colonizing the roots and / or crowns (Figures 1 and 2). Affected tissues, often succulent, tender and therefore very sensitive, are moist and quickly turn brown, rot and die. Some roots, but especially the portion of the stem located on the surface of the soil or substrate, may present a constriction (figure 2). Subsequently, it is quite common to observe foci of seedlings that wilt, turn yellow, wither and / or collapse and more or less disappear (Figures 3).
It should be noted that aerial attacks on seedlings are possible during particularly humid and rainy climatic conditions. Thus, wet and brown lesions can appear on the leaves, petioles and stem and lead to their death. Such symptoms have been reported especially in Florida and associated with P. myriotylum .
The effects of Pythium spp. can persist beyond the nursery, on plants in the development phase after planting and adults. Symptoms resulting from the direct effect of Pythium spp. on the tomato of those linked to their consequences on the proper functioning of plants. The first mainly concern the roots and the crown. In many situations, yellowing , more or less marked and generalized root browning, the disappearance of numerous rootlets are observed. (Figures 4 and 5) Moist and dark lesions may develop at the neck and extend over a few centimeters * ( P. aphanidermatum , P. myriotylum ).
These symptoms can have various consequences: the appearance of symptoms of apical necrosis on the fruits, a reduction in plant growth and sometimes yields, more or less rapid and reversible wilting, drying out, plant death. They are mainly observed in soilless crops, sometimes in soil but in tropical production areas.
Note that a synergy of damage was observed between P. aphanidermatum and the root-knot nematode Meloidogyne incognita .
Especially green and ripe fruits, in contact with or near the ground, can be affected by certain species of Pythium spp. These cause wet lesions which quickly develop into a rot which is the cause of liquefaction and collapse of the fruits. A white cottony mycelium is sometimes visible on the lesions.
Only the observation of oospores (sometimes other structures such as sporangia or chlamydospores) on and in the affected tissues or the realization of isolations on artificial media, make it possible to diagnose with certainty the intervention of these fungi (figures 6 and 7).
For additional information on these Oomycetes, you can consult the sheet Miscellaneous Oomycetes .
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Figure 7