Evaluation of fungicide programs for the control of apple scab on Mcintosh apples in Michigan, 2018

摘要

Fungicides were applied to a block of 17-yr-old 'Mcintosh' trees on EMLA 26 rootstock in East Lansing, MI. Sprays were applied on an approximately 6-8-day schedule until Captan cover sprays, which were applied on an approximately 14-day schedule afterJune 5th. Treatments were replicated four times on single-tree plots and applied dilute (300 gal of water per A) with a handgun at 200-250 psi to runoff. Spray dates and respective growth stages for the treatments are as follows: A — 1 May (1/2-inch green tip); B — 8 May (pink); C — 15 May (full bloom); D — 23 May (petal fall); E — 30 May (fruit set) and F — 5 Jun (1/2 inch fruit). Infection periods and their level of severity (courtesy of www.enviroweather.msu.edu) were identified on: 2-4 May (heavy); 11-13 May (moderate); 15 May (light); 18-23 May (heavy); 30-31 May (moderate); 9-10 Jun (moderate); 22-23 Jun (moderate); 27-28 Jun (heavy); 14-15 Jul (moderate); 15-16 Jul (light); and 21-23 Jul (heavy). The first scab lesions were observed on 14May from the heavy infection period on 2-4 May. From the start of the spray season on 1 May to the end of May, there were 16 days with measurable precipitation that amounted to 4.97 inches. During this period the two prolonged rain events occurred between 11-15 May and 18-22 May. During the first event, 2.5 inches of rain fell over the five-day period leading to a moderate and light infection period. During the second event, 0.92 inches of rain fell over a five-day period leading to a heavy infection period. The month of June had 10 days of measurable precipitation that amounted to 1.44 inches. Many rain events in June did not produce enough wetting hours for a significant infection period. Three rain events occurring on 9 Jun with 0.32 inches, 20 Junwith 0.23 inches and 27 Jun with 0.26 inches produced enough precipitation for three infection periods. Terminal leaf and fruit scab were evaluated on 25 Jun and 31 Aug by examining 20 terminals with about 5 from each tree quadrant and examining a totalof 100 fruit. Inoculum in the block was abundant from the preceding growing-season leading to high disease pressure in this orchard. Fruit from treatments 5, 6, and 7 were harvested to evaluate potential russet. A total of 50 fruit from each treatment repetition were harvested on 13 Sep and evaluated for russet. To assess the level of russet, fruit were evaluated individually and assigned to six categories; 0 — 0-<1% russet, 1 — 1-10% russet, 2 — 11-20% russet, 3 — 2140% russet, 4 — 41-60% russet,and 5 — 61-100% russet. Russet data were then quantified using the 0-5 scale as well as the following russet index equation developed by Holb et al: RI — (NUm/Nt * 0) + (Ncau/Nt * 5.5) + (Ncaa/N * 15.5) + (Ncao/N * 30.5) + (Ncat4/Nt * 50.5) + (Ncat5/Nt * 80.5). RI — the average percentage of fruit surface with russet; Ncat0 Ncat1 Ncat2 Ncat3 Ncat4 and Ncat5 represent the number of fruits in each russet category (Holb et al, Ann. Appl. Biol. 2003, 142:225-233). The coefficients 0, 5.5, 15.5, 30.5, 50.5 and 80.5 represent the median of the lower and upper boundaries from each of the six categories. Data were analyzed using the analysis of variance and LSD mean comparison function of ARM (version 9.3; Gylling Data Management Inc., Brookings, SD). Percentage data were subjected to an acrsine square root transformation before analyses.