There are two things that scare us in the early growing season - frost and hail. Roughly four weeks separates bud break and the last of below-freezing temperatures. While frost events won't kill the vine, it will kill the new shoots that carry the vintage's leaves and fruit. Because wine is so valuable wineries will go to extraordinary measures to prevent mild frost events from destroying an entire year's grape crop such as utilizing massive air moving machines and even low flying helicopters to raise the temperature or at least disrupt the settling of cold air. 2019 saw the second warmest spring on record and we have seen only one minor frost event in Northern Virginia and Maryland which did not appear to have damaged new vine growth.
Hail is much less common than frost events but potentially more damaging. New shoots are delicate and hail can obliterate the leaves, flower clusters, and break the shoot itself off the vine. A client vineyard in 2018 saw massive early spring hail damage that destroyed about 90% of the crop on a 10 acre vineyard. While hail can be damaging at any point in the growing season, new growth is most susceptible in the roughly eight weeks from bud break to fruit set. After that point, the newly formed fruit is small and very firm. The fruit will remain firm until veraison.
Immediately after bud break we need to begin spraying fungicide in the vineyard. So hopefully you have already stocked your first few sprays' worth of chemical before hand. The three least expensive fungicides on the market are copper sulfate, sulfur, and manzozeb. Copper sulfate is primarily used against downy mildew, easily the most pervasive and damaging pathogen affecting grape vines in the mid-Atlantic. Though you would never rely solely on this chemical alone for downy control. Sulfur is used only for powdery mildew control. Powdery mildew is easily controlled with sulfur and because sulfur is so cheap and commonly used, powdery mildew isn't a major concern. Mancozeb is a broad spectrum fungicide (except for powdery mildew control) that is effective against phomopsis cane and leaf spot, a very common early season threat.