Crops
Main symptoms
As we suggested earlier, Oomycetes are likely to cause symptoms on the tomato throughout its development cycle and on several of its organs .
- On seedlings
The Pythium spp. , like Thanatephorus cucumeris (Rhizoctonia solani ), are agents which damping-off well-known cause significant damage in nurseries (Figures 1 to 3). In addition to preventing seeds from germinating, they attack seedlings both pre- and post-emergence. Quite diverse symptoms can be observed depending on the stage of development of the seedlings and the species prevailing:
- wet and brown lesions on the roots, sometimes reaching the crown, causing their decomposition;
- wet and soft changes appearing on the neck and which may extend over a few centimeters. In this case, it gives the impression of having been more or less pinched. The affected tissues gradually turn brown.
Regardless of the primary symptoms, the seedlings soon wilt, collapse and die. They eventually decompose and disappear completely (damping , -off damping-off ).
Several Phytophthora can cause similar symptoms in seedlings or young plants (Figures 4 and 5). For example, P. cryptogea is responsible for brownish to black lesions developing on the stem, at or near soil level, and eventually girdling it. Subsequently, adventitious roots emerge on the upper part of the stem while the roots end up rotting entirely. The leaflets may curl and gradually turn yellow.
Note that in the presence of damping-off, we should keep in mind that an attack by Thanatephorus cucumeris (Rhizoctonia solani ) causes quite similar symptoms.
In addition, several Oomycetes can be responsible for "aerial" lesions ( aerial blight ) on seedlings or adult tomato plants. For example, more or less extensive wet and brown leaf lesions, even rots, have been observed in the United States, Florida, on plants that have been splashed with water following heavy rains. Such lesions were also visible on the petioles and the stem. Pythium myriotylum has been associated with this damage.
- wet and brown lesions on the roots, sometimes reaching the crown, causing their decomposition;
- wet and soft changes appearing on the neck and which may extend over a few centimeters. In this case, it gives the impression of having been more or less pinched. The affected tissues gradually turn brown.
Regardless of the primary symptoms, the seedlings soon wilt, collapse and die. They eventually decompose and disappear completely (damping , -off damping-off ).
Several Phytophthora can cause similar symptoms in seedlings or young plants (Figures 4 and 5). For example, P. cryptogea is responsible for brownish to black lesions developing on the stem, at or near soil level, and eventually girdling it. Subsequently, adventitious roots emerge on the upper part of the stem while the roots end up rotting entirely. The leaflets may curl and gradually turn yellow.
Note that in the presence of damping-off, we should keep in mind that an attack by Thanatephorus cucumeris (Rhizoctonia solani ) causes quite similar symptoms.
In addition, several Oomycetes can be responsible for "aerial" lesions ( aerial blight ) on seedlings or adult tomato plants. For example, more or less extensive wet and brown leaf lesions, even rots, have been observed in the United States, Florida, on plants that have been splashed with water following heavy rains. Such lesions were also visible on the petioles and the stem. Pythium myriotylum has been associated with this damage.
- On adult plants
Of Pythium spp. are frequently isolated roots from apparently healthy or affected tomato . Their presence on a root system is not systematically associated with damage. Indeed, Pythium spp. reveal very different pathogenic powers on tomato roots between species. In addition, the expression of their pathogenic power is often dependent on complex interactions between the development of the host (the tomato in this case), the environment and the Pythium spp. present. Several species are still likely to cause root browning ( Pythium root rot, wilt ) sometimes located at the end of the roots. We note especially the browning, the decomposition and the disappearance of rootlets and roots of small diameter. The cortex of the main roots and the taproot also eventually rot more or less locally. The vascular tissues opposite the altered areas eventually turn brown. These symptoms sometimes appear in soil (especially in humid tropics and subtropics), more frequently in soilless (in temperate and northern production zones) (figure 6).
It should be noted that lightning attacks causing significant plant mortality have been observed in tropical areas, following submersions suffered or caused by tomato crops. These were linked to attacks by P. aphanidermatum in soils with high temperatures, above 30 ° C. It has also been observed that tomato roots tolerate the latter species quite well in the presence of low temperatures. This same species of Pythium is responsible for root rot and crown rot on tomatoes grown in the ground during the summer in Italy.
Various Phytophthora spp. are also reported on tomato roots, also responsible for root and crown rots ( Phytophthora foot and root rot ). The symptoms it induces in these organs are fairly comparable to those caused by Pythium spp., But more severe (Figures 4, 5 and 7).
For example, P. cryptogea is formidable in soilless cultures; it causes root and crown rots, brownish to black. The other species, P. nicotianae (syn. P. parasitica ), P. erythroseptica, P. capsici , cause comparable root degradation in soil and / or soil-less crops. P. capsici is particularly prevalent in the United States in field tomato crops rotating with cucurbits. P. arecae , now classified as P. palmivora , was reported in the Netherlands many years ago to cause root rots and crown cankers.
The root attacks of Pythium spp. like those of Phytophthora spp. (especially in soil-less cultivation) strongly disrupt the functioning of the root system, in particular reducing the absorption of water and nutrients to a greater or lesser extent. They are therefore also accompanied by leaf yellowing and wilting more or less marked and reversible. The fruits of some packages may also have symptoms of necrosis apical ( blossom end rot ). In some situations, the plants show poor growth and reduced fruit size. The expression of leaf symptoms depends on the balance established between the plant's emission of new roots and the proportion of parasitized and damaged roots. Note that the tomato grown in soil, unlike the cucumber, can very well withstand significant root losses without any symptoms of the air being observed.
Oomycetes also attack fruits . The Phytophthora spp. are thus responsible for irregular, moist, greyish-green to brown lesions which appear on the portion of fruit in contact with the ground (figures 8 and 9) to. They evolve rather quickly and generate patterns in concentric rings, complete or incomplete, light brown to dark brown. These lesions are rather firm, smooth on the surface and do not have a well-marked margin. A cottony mycelial felting can cover the lesions in humid conditions ( P. nicotianae, P. capsici, P. drechsleri ). The lesions caused by P. mexicana appear less systematically concentric rings.
Some species of Pythium are the source of wet lesions evolving into rapidly growing rots on mature fruits in contact with the soil; the latter are often associated with discharge and the presence of white cottony mycelium ( P. ultimum, P. acanthicum, P. aphanidermatum ).
Whatever the organ affected and the symptoms observed, we often observe, thanks to a light microscope, the presence of oospores and / or chlamydospores in the altered tissues which make it possible to confirm the presence of one or more Oomycetes. Indeed, it is not uncommon to be confronted with complexes of Oomycetes acting in concert on the roots and fruits of the tomato. This is for example the case in France in soilless crops.
It should be noted that lightning attacks causing significant plant mortality have been observed in tropical areas, following submersions suffered or caused by tomato crops. These were linked to attacks by P. aphanidermatum in soils with high temperatures, above 30 ° C. It has also been observed that tomato roots tolerate the latter species quite well in the presence of low temperatures. This same species of Pythium is responsible for root rot and crown rot on tomatoes grown in the ground during the summer in Italy.
Various Phytophthora spp. are also reported on tomato roots, also responsible for root and crown rots ( Phytophthora foot and root rot ). The symptoms it induces in these organs are fairly comparable to those caused by Pythium spp., But more severe (Figures 4, 5 and 7).
For example, P. cryptogea is formidable in soilless cultures; it causes root and crown rots, brownish to black. The other species, P. nicotianae (syn. P. parasitica ), P. erythroseptica, P. capsici , cause comparable root degradation in soil and / or soil-less crops. P. capsici is particularly prevalent in the United States in field tomato crops rotating with cucurbits. P. arecae , now classified as P. palmivora , was reported in the Netherlands many years ago to cause root rots and crown cankers.
The root attacks of Pythium spp. like those of Phytophthora spp. (especially in soil-less cultivation) strongly disrupt the functioning of the root system, in particular reducing the absorption of water and nutrients to a greater or lesser extent. They are therefore also accompanied by leaf yellowing and wilting more or less marked and reversible. The fruits of some packages may also have symptoms of necrosis apical ( blossom end rot ). In some situations, the plants show poor growth and reduced fruit size. The expression of leaf symptoms depends on the balance established between the plant's emission of new roots and the proportion of parasitized and damaged roots. Note that the tomato grown in soil, unlike the cucumber, can very well withstand significant root losses without any symptoms of the air being observed.
Oomycetes also attack fruits . The Phytophthora spp. are thus responsible for irregular, moist, greyish-green to brown lesions which appear on the portion of fruit in contact with the ground (figures 8 and 9) to. They evolve rather quickly and generate patterns in concentric rings, complete or incomplete, light brown to dark brown. These lesions are rather firm, smooth on the surface and do not have a well-marked margin. A cottony mycelial felting can cover the lesions in humid conditions ( P. nicotianae, P. capsici, P. drechsleri ). The lesions caused by P. mexicana appear less systematically concentric rings.
Some species of Pythium are the source of wet lesions evolving into rapidly growing rots on mature fruits in contact with the soil; the latter are often associated with discharge and the presence of white cottony mycelium ( P. ultimum, P. acanthicum, P. aphanidermatum ).
Whatever the organ affected and the symptoms observed, we often observe, thanks to a light microscope, the presence of oospores and / or chlamydospores in the altered tissues which make it possible to confirm the presence of one or more Oomycetes. Indeed, it is not uncommon to be confronted with complexes of Oomycetes acting in concert on the roots and fruits of the tomato. This is for example the case in France in soilless crops.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9