Candidatus Phytoplasma spp.

 Stolbur, small leaf disease, etc.

General

• Several species of phytoplasmas* affect vegetables in tropical areas, in particular Solanaceae and sweet potato: Candidatus Phytoplasma asteris, C. Phytoplasma solani, etc.
• Their damage is punctual and essentially serious in the open field. 

• Susceptible botanical family(s):

• Production areas affected :

• Organs attacked  :

* Phytoplasmas constitute a fairly recent group (non-cultivable bacteria), and have been classified into 31 distinct species and several sub-species within a constantly evolving classification. They are present in many plants belonging to various botanical families. Note that some plants, less sensitive or of low economic importance, can nevertheless play a major epidemic role as reservoirs of phytoplasmas. The list of host plant species for these microorganisms is still poorly understood.

Biology

• Storage,  phytoplasma reservoirs : 
  • multiply and persist on different cultivated hosts (garlic in New Caledonia) and on weeds, the latter constituting important reservoirs. For example  , queen daisy yellows affect more than 350 different plant species, phytoplasmas of the potato stolbur group infect more than 45 species.
  • These microorganisms also survive in their vectors, several species of leafhoppers.
• Transmission, dissemination :
  • transmitted by several species of leafhoppers in the persistent mode, during feeding bites. As for 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.
  • disseminated over long distances during vector migration. Once in contact with the leaf, these insects puncture the phloem vessels to feed, injecting or taking up phytoplasmas as they pass. The phytoplasma(s), once in the insect, multiply in the cells of the wall of the intestine then cross it. They then reach the haemolymph and from there reach various organs, including the salivary glands, which makes the leafhoppers infectious.
  • transmissible by grafting, by certain parasitic plants, but not by seed in solanaceae and seed in potato.
• Conditions favoring vector flights : hunger, overcrowding, host deterioration, photoperiod, endocrine deficiency in the insect or genetic effects, temperature, wind, etc. Vectors prefer young plants with succulent tissues: in times of drought, they are more likely to move from wild plants to irrigated crops. Hot and dry summers stimulate the migration of some of them. Palatability phenomena are sometimes observed, but they are not well known. Cold winters help reduce overwintering populations.

Protection

• Diseases 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 setting up new crops near other sensitive plants such as eggplant, pepper, potato, tobacco, etc.
• 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, it may be considered to abandon the culture, or even to return it. Otherwise, conduct the crop normally until harvest; be aware that affected plants often produce less.
• The uprooting of diseased plants during cultivation is not of much use because, very often, when the first diseased plants are observed, most of the contamination has taken place and the vector insects have often left to visit other plants.
• At the end of cultivation, carefully eliminate diseased plants , but especially weeds present in the plot or on the outskirts, these may consist of sensitive species serving as reservoir plants.
• The effectiveness of insecticide treatments  is quite controversial: although a certain number of insecticides are very effective against vectors, very often they do not prevent contamination in the field.