Effects of insecticides and repellents on the spread of 'Candidatus Phytoplasma solani' under laboratory and field conditions

Recent outbreaks of 'Candidatus Phytoplasma solani' resulted in severe losses in potatoes, vegetable crops and grapevines in certain regions of Austria and constituted a major challenge for seed potato production. Therefore, the effects of various insecticides and insect deterrents on pathogen spread were studied both in laboratory and field experiments from 2018 to 2021. In laboratory transmission experiments, field captured Hyalesthes obsoletus were caged on differently treated Catharanthus roseus for five days. The insecticides lambda-cyhalothrin, deltamethrin, esfenvalerate, acetamiprid and chlorpyriphos showed the most rapid impact on insect survival and fully prevented phytoplasma transmission. The particle film forming products kaolin and diatomaceous earth had some effect. A transfer of the promising laboratory results to potato fields, however, was achieved to a limited extent only. Treatments with pyrethroids and acetamiprid every 8-10 days over the flight period of H. obsoletus roughly halved the number of symptomatic plants and tubers in case of moderately susceptible varieties and moderate infection pressure. In the event of susceptible varieties and high disease pressure, treatment effects were hardy discernible. In practical terms, the experiments indicate that insecticide applications alone are not sufficient to mitigate the disease. Spraying of diatomaceous earth and mineral oil did not affect disease incidence in the field.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41348-023-00768-y.

Micro-Tom Tomato Grafting for Stolbur-Phytoplasma Transmission: Different Grafting Techniques

Tomato plant, being a model system in scientific research, is widely used to study plant-phytoplasma interaction. Grafting is the faster and most effective method to obtain infected plants. This chapter describes the greenhouse culture of tomato, cv. Micro-Tom, and different herbaceous grafting techniques for efficient stolbur-phytoplasma transmission.

Jasmonate-mediated defence responses, unlike salicylate-mediated responses, are involved in the recovery of grapevine from bois noir disease

BACKGROUND

Bois noir is an important disease of grapevine (Vitis vinifera L.), caused by phytoplasmas. An interesting, yet elusive aspect of the bois noir disease is "recovery", i.e., the spontaneous and unpredictable remission of symptoms and damage. Because conventional pest management is ineffective against bois noir, deciphering the molecular bases of recovery is beneficial. The present study aimed to understand whether salicylate- and jasmonate-defence pathways might have a role in the recovery from the bois noir disease of grapevine.

RESULTS

Leaves from healthy, bois noir-diseased and bois noir-recovered plants were compared, both in the presence (late summer) and absence (late spring) of bois noir symptoms on the diseased plants. Analyses of salicylate and jasmonate contents, as well as the expression of genes involved in their biosynthesis, signalling and action, were evaluated. In symptomatic diseased plants (late summer), unlike symptomless plants (late spring), salicylate biosynthesis was increased and salicylate-responsive genes were activated. In contrast, jasmonate biosynthesis and signalling genes were up-regulated both in recovered and diseased plants at all sampling dates. The activation of salicylate signalling in symptomatic plants might have antagonised the jasmonate-mediated defence response by suppressing the expression of jasmonate-responsive genes.

CONCLUSIONS

Our results suggest that grapevine reacts to phytoplasma infection through salicylate-mediated signalling, although the resultant full activation of a salicylate-mediated response is apparently ineffective in conferring resistance against bois noir disease. Activation of the salicylate signalling pathway that is associated with the presence of bois noir phytoplasma seems to antagonise the jasmonate defence response, by failing to activate or suppressing both the expression of some jasmonate responsive genes that act downstream of the jasmonate biosynthetic pathway, as well as the first events of the jasmonate signalling pathway. On the other hand, activation of the entire jasmonate signalling pathway in recovered plants suggests the potential importance of jasmonate-regulated defences in preventing bois noir phytoplasma infections and the subsequent development of bois noir disease. Thus, on one hand, recovery could be achieved and maintained over time by preventing the activation of defence genes associated with salicylate signalling, and on the other hand, by activating jasmonate signalling and other defence responses.

In-vineyard population structure of 'Candidatus Phytoplasma solani' using multilocus sequence typing analysis

'Candidatus Phytoplasma solani' is a phytoplasma of the stolbur group (16SrXII subgroup A) that is associated with 'Bois noir' and causes heavy damage to the quality and quantity of grapevine yields in several European countries, and particularly in the Mediterranean area. Analysis of 'Ca. P. solani' genetic diversity was carried out for strains infecting a cv. 'Chardonnay' vineyard, through multilocus sequence typing analysis for the vmp1, stamp and secY genes. Several types per gene were detected: seven out of 20 types for vmp1, six out of 17 for stamp, and four out of 16 for secY. High correlations were seen among the vmp1, stamp and secY typing with the tuf typing. However, no correlations were seen among the tuf and vmp1 types and the Bois noir severity in the surveyed grapevines. Grouping the 'Ca. P. solani' sequences on the basis of their origins (i.e., study vineyard, Italian regions, Euro-Mediterranean countries), dN/dS ratio analysis revealed overall positive selection for stamp (3.99, P=0.019) and vmp1 (2.28, P=0.001). For secY, the dN/dS ratio was 1.02 (P=0.841), showing neutral selection across this gene. Using analysis of the nucleotide sequencing by a Bayesian approach, we determined the population structure of 'Ca. P. solani', which appears to be structured in 3, 5 and 6 subpopulations, according to the secY, stamp and vmp1 genes, respectively. The high genetic diversity of 'Ca. P. solani' from a single vineyard reflects the population structure across wider geographical scales. This information is useful to trace inoculum source and movement of pathogen strains at the local level and over long distances.

An abundant 'Candidatus Phytoplasma solani' tuf b strain is associated with grapevine, stinging nettle and Hyalesthes obsoletus

Bois noir (BN) associated with 'Candidatus Phytoplasma solani' (Stolbur) is regularly found in Austrian vine growing regions. Investigations between 2003 and 2008 indicated sporadic presence of the confirmed disease vector Hyalesthes obsoletus and frequent infections of bindweed and grapevine. Infections of nettles were rare. In contrast present investigations revealed a mass occurrence of H. obsoletus almost exclusively on stinging nettle. The high population densities of H. obsoletus on Urtica dioica were accompanied by frequent occurrence of 'Ca. P. solani' in nettles and planthoppers. Sequence analysis of the molecular markers secY, stamp, tuf and vmp1 of stolbur revealed a single genotype named CPsM4_At1 in stinging nettles and more than 64 and 90 % abundance in grapevine and H. obsoletus, respectively. Interestingly, this genotype showed tuf b type restriction pattern previously attributed to bindweed associated 'Ca. P. solani' strains, but a different sequence assigned as tuf b2 compared to reference tuf b strains. All other marker genes of CPsM4_At1 clustered with tuf a and nettle derived genotypes verifying distinct nettle phytoplasma genotypes. Transmission experiments with H. obsoletus and Anaceratagallia ribauti resulted in successful transmission of five different strains including the major genotype to Catharanthus roseus and in transmission of the major genotype to U. dioica. Altogether, five nettle and nine bindweed associated genotypes were described. Bindweed types were verified in 34 % of grapevine samples, in few positive Reptalus panzeri, rarely in bindweeds and occasionally in Catharanthus roseus infected by H. obsoletus or A. ribauti. 'Candidatus Phytoplasma convolvuli' (bindweed yellows) was ascertained in nettle and bindweed samples.