Floral Transmission ofErwinia tracheiphilaby Cucumber Beetles in a WildCucurbita pepo

Abstract

Cucumber beetles, Acalymma vittatum (F.) and Diabrotica undecipunctata howardi (Barber), are specialist herbivores of cucurbits and the vector of Erwinia tracheiphila (E.F. Smith) Holland, the causative agent of wilt disease. Cucumber beetles transmit E. tracheiphila when infected frass falls onto leaf wounds at the site of beetle feeding. We show that E. tracheiphila also can be transmitted via the floral nectaries of Cucurbita pepo ssp. texana L. Andres (Texas gourd). Under field conditions, we found that beetles aggregate in flowers in the late morning, that these beetles chew the anther filaments that cover the nectaries in male flowers thereby exposing the nectary, and that beetle frass accumulates on the nectary. We use real-time polymerase chain reaction to show that most of the flowers produced during the late summer possess beetle frass containing E. tracheiphila. Greenhouse experiments, in which cultures of E. tracheiphila are deposited onto floral nectaries, show that Texas gourds can contract wilt disease through the floral nectaries. Finally, we use green fluorescent protein-transformed E. tracheiphila to document the movement of E. tracheiphila through the nectary into the xylem of the pedicel before the abscission of the flower. Together, these data show that E. tracheiphila can be transmitted through infected frass that falls on or near the floral nectaries. We hypothesize that the concentration of frass from many beetles in the flowers increases both exposure to and the concentration of E. tracheiphila and plays a major role in the dynamics of wilt disease in both wild populations and cultivated squash fields.

Nectar defense and hydrogen peroxide in floral nectar of Cucurbita pepo

“…The presence of yeasts and bacteria in floral nectar was recently highlighted [4,23] and it can be deleterious for plants mainly for two reasons: (i) some of these microorganisms can be phytopathogens and also responsible for the infection of plant tissues [24]; (ii) they use nectar as a growing medium altering the content of sugars and amino acids and thus competing with pollinators for this alimentary resource [25]. To counteract the potentially negative impact of nectardwelling microorganisms, plants evolved defense systems that are mainly based on secondary compounds (such as tannins, phenolics and alkaloids) and proteins.…”

New perspectives in nectar evolution and ecology: simple alimentary reward or a complex multiorganism interaction?

“…The proliferation of microorganisms depends on their ability to tolerate a wide array of nectar environments that may contain different antimicrobial compounds. Some of these microorganisms are phytopathogens deleterious for plant health that exploit nectarostomata (i.e., the specialized open stomata through which nectar is often released) to enter plant tissues [59,60]. Avoiding proliferation of these microbes in nectar greatly reduces risk of infection.…”

Floral traits affecting fire blight infection and management

“…The pathogen gains entry to the inner floral tissues via the nectar secreting stomata, called nectarthodes (Hildebrand and MacDaniels 1935;Rosen 1936;Bubán et al 2003a). The nectary plays an important role also in the transmission of another Erwinia species, E. tracheiphila, which is the causative agent of bacterial wilt disease in cucurbits (Sasu et al 2010a). …”

“…In contrast, Spinelli et al (2007) found that after having reached the nectar cup, E. amylovora moved preferentially inside xylem vessels, although in heavily infected tissues the cortical parenchyma was also colonized. Sasu et al (2010a) also documented the movement of E. tracheiphila through the nectary into the xylem of the pedicel.…”