Jaundice is a disease that affects a large number of dicotyledonous plants (flowering plants) or monocotyledons (grasses). It takes its name from the generalized yellowing of the foliage, which should not be confused with physiological disorders, such as a magnesium deficiency for example. Different microorganisms, mainly viruses, are responsible for this disease. They are all transmitted by very diverse insects, biting-sucking, from the hemiptera family (aphids, whiteflies or leafhoppers).
Description of jaundice
In flowering plants, jaundice is manifested by a premature discoloration of the leaves which thicken, emboss and become brittle. The veins sometimes remain green, but as a rule, the foliage eventually turns completely yellow, then falls off. During flowering, the formation of flowers is disturbed as to their number and size. Often, the flowers present undergo a profound change in the nature of the petals, which turn into small green leaves (this is called virescence). In grasses, barley and winter wheat in particular, the characteristic signs are yellowing of the plants, sometimes accompanied by reddening of the last leaf. In spring, at the supposed moment of growth, the plant remains dwarf irreversibly, the heading is less efficient, which leads to a sharp drop in yields. We speak of dwarfing or dwarfing jaundice in barley and wheat.
Biology and transmission of jaundice viruses
The virus is a microorganism unable to live outside of its host, which is the living cell. Its conservation, in winter, is done in adventitious plants (wild grasses for example), in the eggs of vector insects, or in the salivary glands of the latter, which can survive down to -6 ° C. Viruses can also be present in seeds. Contamination of the plant by viruses is mainly through the action of winged insects. By pricking the conductive tissue of the elaborate sap (phloem), the insect inoculates the virus, the multiplication of which disrupts the metabolism of the plant to its advantage, resulting in slower plant growth. The spread of the disease is related to the movement of insects, from plant to plant and from plot to plot, which makes it essential to monitor the population dynamics of insect vectors.
Plants affected by jaundice
In general, each jaundice virus has its specific vector insect. However, this specificity may prove to be relative, and the same aphid will contaminate several plants (barley, wheat, corn) with different degrees in the severity of the disease. Among the sensitive vegetable plants, one can quote beet, carrot, lettuce, celeriac. In flowering plants, let us quote the asters, coréopsis, queens daisies, poppies… Certain trees, like the laburnum and the hackberry tree, seem to be able to be reached by the viruses of jaundice.
How to fight jaundice?
Direct control of jaundice viruses is impossible. These are combined actions, at different levels, which will limit the harmful effect of these pathogens. First, eliminate the virus reservoirs by removing the weeds from the plots (weeding), and by destroying the various plant waste, either by fire or by removing it from the places of cultivation. Use only seeds previously treated with a systemic insecticide. Avoid bringing certain more sensitive crops closer to others (keep beets and lettuce or barley and corn away). Finally, beyond these common sense precautions, the most important action will be the control of insect vector populations. The intervention threshold will be crossed when at least 10% of the plants are carriers of insects. Chemical insecticide treatment could be specifically considered, starting with the dissemination of warning bulletins from agricultural organizations. By C. Schutz Croué