Effects of Atmospheric & Soil Moisture and Fungicide Physical Mode of Action on the Development of Botrytis Bunch Rot

Latent infections of berries were initiated at late bloom, then fungicides were applied at veraison before infections became active. Some clusters were treated a second time, 15 days later. A single application of Scala or Vangard provided almost complete control of latent infections established 60 days earlier at bloom. Elevate and Rovral were nearly as effective, but allowed a few latent infections to become active within clusters. Flint, Pristine, and Endura provided significantly less control. Following the second application, Scala, Vangard, and Elevate further reduced latent infection frequency to nil, nil, and 0.5%, respectively, but other materials were not similarly affected.

To assess protection of internal berry tissues exposed after injury or cracking, clusters were sprayed through veraison and injected with Botrytis spores2 or 3 weeks later. Scala, Vangard, Rovral, and Elevate provided excellent control, with 96 to 100%of the berries symptomless or showing only limited necrosis around the injection point. Pristine was comparably effective in preventing necrosis, but significantly less effective in limiting sporulation from infections that did occur. Flint and Endura provided the least control, with approximately two-thirds of inoculated berries necrotic and sporulating; sporulation was less intense with Flint.

Fungicides applied 1 day before flowers opened provided no control of infections initiated the next day. However, Elevate, Pristine, and Flint reduced the colonization of cluster debris by 73 to 93%; infected debris within clusters often initiates disease as berries ripen. When inoculated 8 days post-treatment, Elevate and Flint provided significant protective control of berry and peduncle infections; these materials and Vangard also significantly reduced colonization of cluster debris. In contrast, all materials provided excellent post-infection control when applied 3 days after inoculation at either early or late bloom; all materials also provided virtually complete control when berries were sprayed at veraison and Botrytis spores were applied to the surface 2 weeks later.

Individual Pinot noir berries were inoculated at veraison. When symptoms appeared about 1 week later, vines were assigned to one of the three soil moisture regimes: (i) High (soil maintained at ‘field capacity’, all evapotranspired water replenished thrice weekly); (b) Low (vines watered only to alleviate stress when volumetric water content fell below 12%); and (c) Medium (50% of evapotranspired water replaced thrice weekly). Relative to clusters in the ?Low? treatment, disease spread was approximately twice as great in the ?Medium? regime and three times as great in the High.

Bottom line:

  • All fungicides provided excellent protective control against late season berry infections. However, only Flint and Elevate provided consistent protective control of infections initiated at bloom.
  • Scala, Vangard, Elevate and Rovral provided excellent suppression/eradication of latent infections established at bloom when applied 60 days later at veraison; this activity was further improved by a second application 15 days later. These materials also provided excellent protection of internal berry tissues. Flint, Pristine, and Endura were significantly less effective in both manners, but all materials provided excellent post-infection activity at bloom.
  • Preharvest disease spread is more severe in well-watered vines than in those growing in dry soils.