Yellow, chlorotic or bleached leaves

The different types of symptoms

The yellowing of leaves, also called chlorosis, is a frequently observed symptom in tobacco and not really a specificity of a given disease. It may indeed show many different aspects:
- limited to a small area in the form of a spot (figure 1) or associated to a macula surrounding it with a more or less marked yellow halo (figure 2);
- affecting only one side of a leaf, this unilateral yellowing often characterises vascular diseases (figure 3). For a more accurate diagnosis, have a look at the section on External or internal stem lesions;
- developing from the veins (figure 4) or between them (figure 5, interveinal chlorosis).
 

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

 
The phenomena of chlorosis may start with the young apex leaves or old leaves from the base of plants, in some cases the middle leaves show the chlorotic symptoms. Sometimes, the yellowing can be generalized to the entire plant. It may take very different intensities varying from light green to bright yellow, sometimes progressing to leaf bleaching. In some cases, the whole leaf areas can be completely bleached, tissues are devoid of pigments (chlorophyll, carotenoids and lycopenes).
 
  Leaf chlorosis is a marker symptom of a plant malfunctioning, resulting frequently from:
- one or more parasitic attacks occurring either directly and locally on the leaf (e.g. airborne disease), or on other plant parts, especially on the roots or the stem;
- non-parasitic diseases such deficiencies or chemical injuries. 
 It is often difficult to determine the cause of yellowing: so be very careful when making your diagnosis.

Various causes may be responsible for leaf chlorosis in tobacco, they can be

•  either parasitic such as :

- fungal and bacterial parasites of roots, collar, stem and tobacco vessels (see the sections concerned, especially the fact sheet on Vascular Diseases);
- several viruses are capable of inducing more or less pronounced chlorosis, see the fact sheet on Leaf Mosaics;
- phytoplasms responsible for Stolbur 
 

•  or non-parasitic such as :

-  polyphylla or ("Frenching")
- nutritional  disorders (deficiency, poisoning)
- various chemical injuries
- genetic abnormalities
 

Figure 1	Figure 2	Figure 3

A limited number of additional features allows  us to distinguish the major diseases that cause yellowing and chlorosis in tobacco leaves.: 
- Stolbur is common in some areas, but usually only a few plants are affected. Affected plants are often distributed randomly in the plot and the first symptoms appear on lower and intermediate leaves (figure 1). In some years, during the massive flights of vector insects (leafhoppers), the number of affected plants may be important and approach 100%;
 - Polyphylla is a rare disease, few plants are affected and they are distributed around a number of focal points. Symptoms can be observed on upper leaves; 
- nutritional disorders are uncommon but they often affect many plants spread over the whole plot. Depending on the mineral deficiency or excess, symptoms are visible in either the upper or in the lower leaves;
 - genetic abnormalities (albinism) are rare and affect very few plants, usually one plant distributed randomly in the plot. These plants are affected in upper or lower leaves (Figures 2 and 3) should be considered as curiosities (see the fact sheet of Leaf mosaic ).