Candidatus  Phytoplasma spp.

 Stolbur, small leaf disease, etc.

General

• Several species of phytoplames * affect vegetables, in particular Solanaceae: Candidatus Phytoplasma asteris, C. Phytoplasma solani, etc.
• Their damage is punctual and essentially serious in the open field.
• Eggplant does not appear to be frequently affected.
• Organs attacked  : leaves, flowers, fruits, stems

* Phytoplasmas are a fairly recent group (non-cultivable bacteria), and have been classified into 31 distinct species and several subspecies within a constantly evolving classification. They are present in many plants belonging to various botanical families. It should be noted that certain plants, less sensitive or of low economic importance, can nevertheless play a major epidemic role as reservoirs for phytoplasmas. Little is known about the list of plant species that host these microorganisms.  

• Symptoms :
  • Chlorotic plants with more or less slowed growth, with apices with shorter internodes (photos 1 and 2).
  • Distorted and discolored leaves: smaller, mostly yellow, sometimes and / or purplish (anthocyanin colouration) (figure 3).
  • Leaf blade sometimes thicker.
  • Abnormally upright flowers, often sterile with morphological aberrations:
    • The sepals, whose veins take on a purplish color, remain completely joined and the calyx is enlarged (big bud) (figure 3);
    • Rare fruits with reduced growth.

• >>> More pictures
• Signs  : no visible sign, confirm the possible presence of vectors.
• Possible confusion : herbicidal phytotoxicity, etc.
• Phytoplasmas reported on eggplant :

Biology

• Conservation,  phytoplasma reservoirs : 
  • Multiply and perpetuate themselves on different cultivated hosts and on weeds, the latter constituting important reservoirs. For example, the  yellows of the daisy affects more than 350 different plant species, the phytoplasmas of the potato stolbur group infect more than 45 species.
  • These microorganisms are also preserved in their vectors, several species of leafhoppers.
• Transmission, dissemination :
  • Transmitted by several species of leafhoppers in the persistent mode, during feeding bites. As with the hosts of these phytoplasmas, the number of vector leafhopper species is large and fluctuates according to the phytoplasmas. 
    • Candidatus Phytoplasma stars: 30 species of cicadelles dont Macrosteles spp., Euscelis spp., Scaphytopius spp., Aphrodes spp., Orosius argentatus , Euscelidius variegatus , etc.
    • Candidatus  Phytoplasma solani (figure 1) :  Hyalesthes obsoletus  Signoret, etc.
  • • Other insects of the same family have been reported as vectors: Hyalesthes mlokosiewiczi, Pentastiridius leporin, etc.
  • Scattered over long distances during vector migration. Once in contact with the leaf, these insects bite the phloem vessels to feed, injecting or removing phytoplasmas as they pass. The phytoplasma (s), once in the insect, multiply in the cells of the wall of the intestine and then pass through it. They then reach the hemolymph and from there reach various organs, including the salivary glands, making the leafhoppers infectious.
  • Transmissible by grafting, by certain parasitic plants, but not by seeds in nightshades and seeds in potatoes.
• Conditions favoring vector theft : hunger, overcrowding, host deterioration, photoperiod, endocrine deficiency in insects or genetic effects, temperature, wind, etc. Vectors prefer young plants to succulent tissues: in times of drought, they are more likely to switch from wild plants to irrigated crops. Hot and dry summers stimulate the migration of some of them. Appetite phenomena are sometimes observed, but they are poorly understood. Cold winters help reduce winter populations.

Protection

• Diseases that are very difficult to control, even more so once symptoms have appeared in the crop: an infected plant will remain so throughout its life.
• Protect the plants in the nursery with an agro textile (non-woven veils, mesh fabrics) which will constitute an effective mechanical barrier.
• Use healthy plants, and avoid planting new crops near other sensitive plants such as tomatoes, peppers, potatoes, tobacco, etc.
• Carefully weed nurseries, plots and their surroundings (edges of hedges and paths, etc.)
• Aluminized mulch could reduce the number of vectors and the incidence of phytoplasmosis.
• If too many plants are affected, consideration may be given to abandoning the crop or even turning it over. Otherwise, lead the crop normally until harvest; be aware that affected plants often produce less.
• Pulling up diseased plants during cultivation is not of much use because, quite often, when the first diseased plants are observed, most contaminations have taken place and the vector insects have often left to visit other plants. plants.
• At the end of cultivation, carefully eliminate the diseased plants, but especially the weeds present in the plot or on the periphery, these can be made up of sensitive species serving as reservoir plants.
• The effectiveness of insecticide treatments is quite controversial: while a certain number of insecticides are very effective against vectors, very often they do not prevent contamination in the field.