Root yellowing and browning

The majority of parasitic or non-parasitic diseases affecting tobacco roots cause diffuse yellowing, browning (localised or generalised), necrosis and disappearance of many roots and rootlets. In severe cases, the root system is completely damaged and vessels located at the pivot may turn slightly yellow or brown (figures 1-5). The lesions spread occasionally on collar and stem basis, causing rather serious cankers close to soil line (see the fact sheet on the Various collar and/or stem lesions )

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

Among the parasitic causes of root yellowing or browning, are:

 - bacteria such as Agrobacterium tumefaciens, which has never been observed in France, and Ralstonia solanacearum (bacterial wilt). The latter, after having sufficiently progressed in tobacco vessels, extends to the cortex of the stem and collar, and also to primary and secondary roots. The roots  turn brown to black and rot. We advise you to refer to the fact sheet of Internal stem lesions for better understanding of this bacteriosis;

 - fungi, such as Calonectria Crotalarieae (formerly Crotalarieae Cylindrocladium), Diaporthe ziziphina Speg. (1909), Helicobasidium mompa (see the fact sheet on Other parasitic fungi of the root ), Pythium spp. (you can also see Damping-off of seedlings in the nursery), Thanatephorus cucumeris (Rhizoctonia solani) and Thielaviopsis basicola (formerly Chalara elegans);

- nematodes, such as Globodera tabacum and Pratylenchus spp.
 
The most common non-parasitic causes are waterlogging or hardpan.

Note that a number of diseases, in addition to symptoms described above, induce relatively characteristic symptoms that make their identification easier. Others are more difficult to identify. This  is why laboratory tests are often necessary to define the cause(s). Among them, for example, infections by Rhizoctonia solani or those by various Pythiaceae, or erroneous agricultural practices. The latter can lead to root suffocation in case of excess water, burning of rootlets when a very localised or high rate of fertiliser is applied.

Often these symptoms occur together, or more precisely, some diseases make plants more susceptible to other injuries. For example, infections by Pythiaceae or T. basicola may be more severe in heavy, poorly drained soils and asphyxiated (watelogged) plants are more susceptible to pathogens.
 
When root damage occur do not hesitate to question yourself and / or producer!

Sometimes simply by answering the following questions, one  may be able to understand the nature of the problem (an affirmative answer to any of these questions will help to confirm the hypothesis)
- Did you give too much water to your plants?
- Are the diseased plants located in the wettest places of the field?
- Did you plant when soils were still cold and wet?
- Did you irrigate too much in nursery and when planting?
- Is the soil well drained?
- Did you apply too much fertiliser before planting or during the growing season?