Biology, epidemiology

• Conservation, phytoplasma reservoirs
  

The phytoplasmas responsible for the yellows of the daisy, stolbur, etc., are rather polyphagous organisms which can multiply on different cultivated hosts and on weeds , the latter constituting important reservoirs. The host range appears to vary from one phytoplasma to another.

The yellows of the queen-daisy affect more than 350 different plant species , cultivated or not, distributed in about fifty botanical genera. Several weeds and wild plants can harbor phytoplasmas: clover, Salsola tragus , several species of Plantago and Sonchus (including S. asper ), Taraxacum officinale , wild lettuce, Senecio cruentus, Argyranthenium frutescens, Spartium junceum ... Some are also reported on potato, eggplant, pepper, corn, Ipomoea obscura, salad, carrot, spinach, celery, zucchini, Brussels sprout, onion, broad bean, dahlia, Hydrangea macrophylla, Lavandula officinalis , olive tree, Primula sp., Viola odorata …

The phytoplasmas of the potato stolbur group infect more than 45 species among Solanaceae, and at least 16 species belonging to 6 other botanical families. They multiply in these plants which sometimes serve as sources of inoculum. Among the cultivated plants, let us retain the pepper, the eggplant, the potato, the celery, the carrot, the strawberry, the grapevine, the tobacco, the lavender, the avocado…. Among weeds, we can cite bindweed ( Convolvulus arvensis and Calystegia sepium , see photo 881), black nightshade (photo 882), nettle ( Urtica dioica ), clover… Bindweed and nettle seem to play a crucial role in the epidemiological cycle of stolbur as preferred hosts of one of the potential insect vectors, strongly influencing epidemics of stolbur.

Many other hosts have been suspected in several countries of being potential reservoirs of phytoplasmas or “dead end” hosts (without knowing their specific group) in the context of phytoplasmosis epidemics on tomatoes: carrot, pepper, Endive, strawberry, avocado, pear… (in Spain), Artemisia absinthium, Cirsium arvense, Cichorium intybus, Convolvulus arvensis, Taraxacum officinale … (in Russia), pepper, tomato, tobacco, carrot, parsley, celery, grapevine, turnip, Datura stramonium, Taraxacum offinale, Silene vulgaris (in Hungary), eggplant, pepper, Cryptotaenia japonica, Chrysanthemum coronarium, Gentiana sp.… (in Japan). We should especially beware of bindweed and nettles.

These phytoplasmas are also conserved in their vectors , several species of leafhoppers. The cycle of these insects involves eggs which play no role in the winter conservation of the phytoplasma but ensure the durability of the insect from one season to another.

• Transmission, dissemination
  

As we mentioned previously, these phytoplasmas are 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 asteris » : 30 espèces de cicadelles dont Macrosteles spp., Euscelis spp., Scaphytopius spp., Aphrodes spp., Orius argentatus, Euscelidius variegatus …

“ Candidatus Phytoplasma solani” (figure 1): Hyalesthes obsoletus Signoret (photo 880) would be the most important vector in europe. This leafhopper from the family Cixiidae is a polyphagous species vector of the stolbur phytoplasma of group 16SrXII-A, also present on Convolvulus arvensis, Urtica spp., Ranunculus spp., Senecio spp. and Artemisia spp., more rarely on woody fruit plants and on grapevines… The imagos are active from May to mid-August, with one generation per year and one overwintering in larval form.

On the other insects of the same family have been reported as vectors Hyalesthes mlokosiewiczi, Pentastiridius leporinus ...

The leafhoppers , piercing-sucking insects disseminate phytoplasma over long distances during migration and contaminate late tomato plants spring and summer (late June, in July in France). Once in contact with the leaf, they 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. Macrosteles quadrilineatus can remain so for at least a hundred days.

These insects are usually occasional on infected plants. They are able to fly long distances or stay put. The date of onset of symptoms, which generally occurs 30-45 days after contamination, depends on the period of migration of the vector (s). Migration is a complex phenomenon which consists of the transfer of insect populations from place to place in the form of mass flights. The causes, poorly understood, seem to be linked to local conditions unfavorable to leafhoppers. Among the factors that influence migration and the nature of flights, we can cite hunger, overcrowding, host deterioration, photoperiod, endocrine deficiency in insects or genetic effects, temperature, wind, etc. Insects prefer young plants to succulent tissues: in times of drought, they are more likely to switch from wild plants to irrigated crops. Hot, dry summers stimulate the migration of some vectors. Appetite phenomena are sometimes observed, but they are poorly understood. Cold winters help reduce winter populations.

These phytoplasmas are transmissible by grafting . Different species of ( dodder Cuscuta campestris, C. epilinum, C. trifolii, etc.), parasitic plants rife in particular on tomatoes (see page), are capable of transmitting phytoplasmas, and in particular those responsible for stolbur. Note that phytoplasmas have been found in broomrape ( Orobanche aegyptiaca ) parasitizing tomato roots, suggesting that they could contribute to their transmission. They do not appear to be seed-transmissible in Solanaceae. They would not be by potato seeds either.