Aspects of resistance to Tetranychus urticae in New Zealand strawberry cultivars

Abstract

Resistance to Tetranychus urticae in four strawberry cultivars (Redgauntlet, Sequoia, Cambridge Favourite, Tioga) was investigated. The mite responses to the plant (antibiosis and preference/non-preference), and the plant responses to the mite (tolerance), were evaluated. Monitoring of natural T. urticae populations in the field over three seasons, and comparative studies on plants sprayed at specified mite density treatment levels, indicated very similar degrees of overall cultivar acceptability to the mite. Laboratory life table studies demonstrated that egg hatch, immature survival and development rate, and longevity were very similar on the cultivars studied. This suggested a lack of physical or chemical properties causing antibiosis. The farnesol content of foliage, which has been claimed as an indicator of susceptibility to the mite, was investigated. No farnesol was detectable in any of the cultivars tested, and the results suggested that this chemical was misidentified in earlier published investigations. In the laboratory, preference/non-preference studies measured the adult acceptance of a cultivar as a host, and the ovipositiona1 response to cultivars. Results from acceptance trials were contradictory with no overall ranking of the cultivars possible in terms of their acceptability to the mite. The ovipositional responses were consistent, but with no significant preferences shown for one cultivar over another. A "choice" technique evaluated these preferences on two cultivars at a time, and the validity of the techniques was confirmed over a wider range of cultivars, some with reported resistance. The results confirmed resistance in the latter, compared to susceptibility of Tioga and Redgauntlet. Preference/non-preference, rather than antibiosis, appeared to be the principal component of the variation in mite response to the various host cultivars. A phenomenon of greatly reduced mite infestations on plants in their second season compared to first year plants was investigated. Population densities on two year old field plants were consistently lower and peak numbers later than on one year old plants in the same season. These observations were supported by laboratory feeding and oviposition trials. Tolerance studies assessed inter-cultivar differences in leaf damage, crop yield and quality under different mite infestation regimes. Leaf damage in field samples was very consistent for one year old plants, with Sequoia having the least damage per mite-day per leaf, and Redgauntlet generally the greatest. No conclusions were possible on the relative leaf damage to two year old plants of these cultivars. Sequoia was the most tolerant of the four cultivars in terms of crop yield response to mite infestation, and Redgauntlet was the least tolerant. Fruits from two year old plants were smaller than those from one year old plants, and more numerous. In addition to supporting fewer mites on the leaves, two year old plants of all cultivars were far less mite-tolerant in crop yield response than one year old plants. The effects of mite infestations on the berry were assessed by sensory evaluation trials. Sequoia berries were not significantly affected by increasing mite densities. For the other cultivars, an increase in accumulated mite-days per leaf caused a decline in the acceptability of the berries. Fruits from two year old plants were equally acceptable to the consumer as those from one year old plants. From the yield and quality studies, a threshold for miticide application to one-year old plants was suggested of 100 mites per leaf for Sequoia and Tioga, and between 25 and 100 mites per leaf for Cambridge Favourite and Redgauntlet. Intercultivar differences in this study were largely in plant tolerance to mite infestations, rather than mite preference/non-preference or antibiotic responses to the four host cultivars.