Role of Metcalfa pruinosa as a Vector for Pseudomonas syringae pv. actinidiae

Abstract

After 20 years of steady increase, kiwifruit industry faced a severe arrest due to the pandemic spread of the bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa). The bacterium penetrates the host plant primarily via natural openings or wounds, and its spread is mainly mediated by atmospheric events and cultural activities. Since the role of sucking insects as vectors of bacterial pathogens is widely documented, we investigated the ability of Metcalfa pruinosa Say (1830), one of the most common kiwifruit pests, to transmit Psa to healthy plants in laboratory conditions. Psa could be isolated both from insects feeding over experimentally inoculated plants, and from insects captured in Psa-infected orchards. Furthermore, insects were able to transmit Psa from experimentally inoculated plants to healthy ones. In conclusion, the control of M. pruinosa is recommended in the framework of protection strategies against Psa.

Keywords: bacterial canker of kiwifruit; insect-mediated disease transmission; microscopical visualization.

Fig. 1

Experiment set up: (A) Artificial feeding of Metcalfa pruinosa. Individual insects, closed in an isolated chamber, feed on a Psa-inoculated solution (in grey). The population of Psa in the insect is subsequently determined. (B) Transmission of Psa from M. pruinosa adults, fed with a medium containing the bacterium, and subsequently transferred on healthy kiwifruit leaves. (C) Transmission of Psa from M. pruinosa adults, fed on experimentally inoculated kiwifruit plants, and subsequently transferred on healthy ones.

Fig. 2

(A) Percentage of infected insects in relation to artificial feeding time. The correlation was significant according to Fisher’s exact test. (B) Population of Pseudomonas syringae pv. actinidiae in the contaminated insects, expressed according to the days of artificial feeding. The data refer to 12–20 individual insects per each sampling point (A) and to those positive for Psa among them (B) in the 2013 and 2014 artificial feeding experiments. Bars with different letters are significantly different according to Fisher’s LSD test (P < 0.05).

Fig. 3

Bacterial load in relation to insect weight. Data refer to average values with standard errors of contaminated insects (5–9 per point) taken at the same sampling times in the 2013 and 2014 artificial feeding experiments. The correlation is significant according to Fisher’s exact test (P < 0.05).

Fig. 4

Fluorescence steromicroscopy photographs of healthy (A) or contaminated (B) Metcalfa pruinosa adult individuals. Bright green fluorescent emission is due to Pseudomonas syringae pv. actinidiae (CFBP7286-GFPuv) colonization of tissues (1: mouthparts, 2: limbs and 3: urosternites). Insects were contaminated by feeding on artificial medium containing the GFPuv-labelled pathogen. Photographs were taken after 4 days of feeding.

Fig. 5

Presence of Pseudomonas syringae pv. actinidiae in Metcalfa pruinosa 2014 field captures. Insects (24 for each class) were captured from a kiwifruit orchard located near Faenza (Italy) in mid-June (juveniles) and at the end of July (adults). (A) percentages of infected insects. (B) Pseudomonas syringae pv. actinidiae population per insect. Standard error is shown.

Fig. 6

Population of Pseudomonas syringae pv. actinidiae in host plants, in relation to the number of vector insects feeding on them. The correlation is significant according to Fisher’s exact test with a confidence level of 0.05.