New Zealand flower thrips as a vector of Monilinia fructicola (Wint.) honey in peaches in Canterbury, New Zealand

The New Zealand flower thrips, Thrips obscuratus (Crawford) (Thysanoptera: Thripidae), is an important seasonal pest of peaches infesting flowers during spring and mature fruit prior to, and during fruit harvest. Thrips numbers were monitored during peach flowering and prior to fruit harvest in a 160 tree experimental orchard, using water traps, sticky traps on tree limbs, and by direct sampling of flowers and fruit. Thrips numbers in water traps in the spring were low compared to summer samples. Adult thrips were found in peach flowers from pink to shuck fall with peak numbers during full bloom. Relatively low numbers of thrips were caught on sticky traps placed on tree stems in the spring which supports the notion that thrips numbers in the spring are low. Very high numbers of thrips ( > 1,000 thrips/ trap/ week) were caught in water traps prior to, and during fruit harvest. Adult thrips were found on peach fruit from three weeks prior to harvest with peak numbers at full fruit ripeness. During rain thrips moved to protected areas on the bottom of the fruit presumably to avoid being washed off the fruit. The association between thrips and Monilinia fructicola (Winter) Honey inoculum sites was studied using sticky traps placed near twig cankers and fruit mummies, and by counting thrips numbers on brown rot diseased sporulating fruit. Adult thrips were caught on sticky traps placed near brown rot infected twig cankers in the spring and summer, and on sticky traps placed near brown rot infected fruit mummies in the spring. Spring sticky trap catches showed a similar trend to water trap catches with peak thrips numbers caught in traps when the flowers were at or near shuck fall. Summer sticky trap catches were much higher than spring catches. Thrips were found on brown rot infected sporulating peach fruit in comparable numbers to thrips found on healthy peach fruit. Peak numbers of adult thrips were found on diseased fruit with a low sporulation severity. At high sporulation severities few thrips were found on diseased fruit. A thrips-washing method was developed and viable M. fructicola spores were enumerated on thrips removed from twig cankers, fruit mummies, and fruit exhibiting sporulating brown rot lesions. Two methods were developed to follow thrips dispersal of M. fructicola in and between peach trees in the spring and summer. Feeding thrips rubidium and assaying thrips for rubidium content was feasible in the laboratory, but due to the high number of thrips required for field experiments and associated difficulties in feeding large numbers of thrips rubidium, the method was not suitable for following thrips dispersal in the field. Thrips treated with spores of a fungicide-resistant strain of M.fructicola dispersed spores to flowers and fruit in the field with disease resulting. In all experiments most fungicide-resistant strain diseased flowers and fruit were found on the tree the thrips had been released onto, followed by trees in the same row as the release tree, and then across tree rows. Controlled environment studies showed increasing brown rot disease incidence and severity in flowers with increasing numbers of thrips per flower and increasing spore loads on the thrips. Thrips deposited spores onto fungicide-treated fruit which resulted in disease, thereby apparently negating the benefits of fungicide protection. A field experiment was conducted to compare the incidence of brown rot disease in peaches at harvest in the presence and absence of thrips. Brown rot disease incidence on fruit at harvest where thrips were excluded by using an insecticide was significantly (P < 0.05) lower than on fruit where water was applied and thrips numbers were higher. Laboratory experiments showed that thrips did not predispose flowers to infection and disease as thrips applied as a pre-treatment to flowers prior to M. fructicola spore application did not increase disease incidence. A possible explanation for this was the high susceptibility of flowers to infection. A single spore application method was developed and single spores were found to act independently in the infection process, with only one spore required for flower infection. There were differences found in the susceptibility of flower parts to infection with filaments being the most susceptible to infection followed by anthers, stigmas, and petals. The practical implications of thrips dispersal of M. fructicola were discussed as thrips were shown to be an important factor in brown rot epidemiology in Canterbury peach orchards.