Applications of the MARYBLYT computer model for identifying infection risk for fire blight of apple

Abstract

The MARYBLYT computer model, Version 4.1, was used to identify fire blight blossom infection events on Royal Gala apple trees for the springs of 1993-1995 in Hawke's Bay, Nelson and Waikato. Over the three seasons, only one mid bloom infection was predicted in 1993 in a Hawke's Bay orchard. For all regions, several infection events were identified towards the end of flowering, but most of them occurred after petal-fall. Spray diary analysis indicated that streptomycin sprays that were applied during early to mid flowering were not targeted for the infection periods. INTRODUCTION Fire blight is a bacterial disease caused by Erwinia amylovora. The pathogen infects many Rosaceous plants including apple, pear, hawthorn, cotoneaster and pyracantha. Fire blight infects blossom, fruit, shoots and branches. With blossom blight, flowers turn black or brown and the whole flower cluster droops. The development of blossom blight is related to temperature because multiplication of E. amylovora is governed by temperature. Rain or moisture plays a part in spreading the bacteria from the stigmas, site of bacterial multiplication, to the nectarial cells where infection occurs (Thomson 1986). Since blossom infections are primarily influenced by weather, the risk of infection can be identified by monitoring temperatures and rainfall. A computer model MARYBLYT (Lightner and Steiner 1990) has been developed to predict infection periods and is used to assist timing of control of fire blight in America (van der Zwet et al. 1994). The predictive criteria of both the MARYBLYT and the Thomson and Schroth (Thomson et al.1982) models were used to develop a prototype of a fire blight predictor in New Zealand (Manktelow 1993). Both the MARYBLYT model and the fire blight predictor have been trialed by growers and researchers (Manktelow, Tate pers. comm.). The MARYBLYT model has been evaluated for prediction of blossom blight in New Zealand since 1993 and has been shown to be accurate for predicting blossom blight on Royal Gala and Braeburn apple trees (Gouk et al. 1994, 1995). This paper focuses on the use of MARYBLYT for identification of blossom blight infection events for Royal Gala apple cultivar in three apple growing regions. METHOD Predictive criteria of MARYBLYT MARYBLYT predicts disease through recognition of weather conditions suitable for infection during flowering. The conditions governing infection of blossom in MARYBLYT are: (a) Presence of open flowers with stigmas and petals intact; (b) Accumulation of at least 110 degree hours > 18.3 o C after first bloom; (c) Occurrence of dew or • 0.25 mm rain, or • 3 mm rain the previous day; (d) An average daily temperature of • 15.6 o C.