Influence of Grape Berry Age on Susceptibility to Guignardia bidwellii and Its Incubation Period Length

Abstract

The period of fruit susceptibility to Guignardia bidwellii (anamorph Phyllosticta ampelicida), the causal agent of grape black rot, was determined in the field. Intact fruit were inoculated weekly from bloom until 8 weeks later with a suspension containing 2 x 10(5) conidia per ml. Disease progress was monitored approximately every 2 days until 3 to 5 weeks after inoculation, depending on the year and variety. Fruit of Vitis x labruscana 'Concord' exhibited a period of maximum susceptibility from midbloom until 2 to 4 weeks later, although some berries became symptomatic when inoculated 4 to 5 weeks postbloom. Fruit of V. vinifera 'Chardonnay' and 'Riesling' exhibited a period of maximum susceptibility from midbloom until 3 to 5 weeks later, although some berries retained their susceptibility until 6 to 7 weeks postbloom. These susceptible periods were approximately 2 to 4 weeks shorter than previously assumed. Fruit age at the time of inoculation affected the length of the incubation period (time from inoculation until symptom appearance). When the incubation period was defined in terms of degree hours (base = 0 degrees C) accumulated after inoculation, DH(50) values (the number of degree hours required to reach 50% of final disease severity) increased by at least 50% as berries neared the end of their susceptible period. Newly symptomatic berries continued to appear for over 1 month after inoculation of older fruit. Thus, age-related or ontogenic, host resistance was manifested as both a decline in susceptibility and a significant increase in incubation period length. The control of black rot is likely to be improved by tailoring the intensity of fungicidal protection to the abbreviated period of fruit susceptibility defined in this study. Furthermore, the efficacy of management programs and the results of epidemiological studies are likely to be misinterpreted unless the variable effect of fruit age on incubation period length is recognized.

A Cumulative Degree-Day-Based Model to Calculate the Duration of the Incubation Period of Guignardia bidwellii

“…)-NO correction for the upper threshold (24°C in the present model) was possible becatise no daily-based average temperatures were given by Hoffman et al (5). The CDDTcLusTER(6;24) values were calculated by multiplying the CDDTLEAF{6;24) (175 CDDs) with the correction factor c (phenological stage at inoculation in the trials of Hoffman et al [5]). Basis for the correction factor is the CDD>io value at moment of inoculation.…”

“…The validation of the model on clusters was made with the data given by Hoffman et al (5) for inoculations 3 to 7 weeks after fiowering in the year 2000 obtained in Riesling in Geneva, NY. The given data ( Table 4 in Hoffman et al [5]) for cumulative degree-hours above 0°C (CDHUMQO;.))…”

“…The given data ( Table 4 in Hoffman et al [5]) for cumulative degree-hours above 0°C (CDHUMQO;.)) were divided by 24 to calculate the CDD (lower threshold; 0°C) (CDD|NC(O;-))-TO adjust this CDD to the present model, the CDDINQO;.)…”

“…This CDD>io value was calculated by dividing the accumulated degree-hours after 50% bloom given by Hoffman et al (5) in Table 2 by 24 (hours) and 10 times the number of days post bloom at which the inoculation took place. Contrary to the calculations in the present model, the start of summation for the CDD>io value was, in case of the validation based on the data of Hofftnan et al (5), not BBCH 68 but BBCH 65 (8) because data given were for this developmental stage (Table 4). Calculated CDDINQÔ;-) values were plotted against the calculated CDDTCLUSTER(6;24) values.…”

“…On clusters, examinations of the duration of the incubation period were conducted by Northover (16) and Hoffman et al (5). The latter found that the duration of the incubation period is affected not only by ¿he temperature conditions but also by the phenological growth stage.…”