http://ohioline.osu.edu/hyg-fact/3000/3025.html

"Berries that escape bloom-time infection may become infected at or near harvest under favorable environmental conditions. Any wound on the berry provides an excellent infection site for the fungus even in the absence of favorable environmental conditions. Birds, insects, hail, and powdery mildew are common causes of wounds. Swelling during ripening in tightly packed clusters causes pressure that also can rupture the berries. Wet and humid conditions around the berries and leaves greatly enhance disease development. The longer wet conditions persist, the greater the probability of infection, even to undamaged berries. Warmer temperatures also favor infection. At 54 to 75 degrees F, infection occurs in 12 to 24 hours, while at 37 degrees F, 60 to 72 hours are required."

http://winegrapes.tamu.edu/grow/diseases/bunchrot.shtml

"Any injury to the skin of ripening berries, from large to very small, is a potential entry point for these pathogens. Bunch rot can quickly develop in wounds resulting from bird feeding, hail injury, rain splitting, and mechanical cracks or fruit abscission (separation from the pedicel) caused by growth pressure in tight-clustered grape varieties. Small entry holes created by grape berry moth (Endopiza viteana) larvae are a common cause of bunch rot in Texas vineyards. Other insects and diseases can cause much less obvious injury to the fruit that nonetheless provides entry points for bunch rot pathogens. Infection by the powdery mildew fungus creates lesions that may later cause skin cracking leading to bunch rot, but even inconspicuous powdery mildew infections can increase the severity of bunch rot. Similarly, early season feeding injury from tiny thrips causes scaring on fruit skin that reduces its elasticity, resulting in small cracks as the berry grows."

http://www.grapes.msu.edu/botrytis.htm

“Botrytis bunch rot is a fruit rot, but it also can affect other plant parts. In spring, buds and young shoots may be infected and turn brown. In late spring, V-shaped or irregular brown patches may appear on leaves. Inflorescences may become blighted and wither away. Some flower infections remain latent until veraison. From veraison onward, the fungus can infect grape berries directly through the epidermis or through wounds. Compact clusters, powdery mildew infection, hail and insect damage (e.g., grape berry moth) can predispose grapes to infection."

http://www.ipm.ucdavis.edu/PMG/r302100111.html

“The fungus overwinters as sclerotia in berry mummies on the ground or left hanging on the vine and in canes. Germination and spore production occur in spring. Infections require free water for a definite period of time depending on temperature. Infections may occur during bloom should rains occur; preclose rachis infections often occur on Chardonnay. Late-season infections are most severe when relative humidity exceeds 92%, free moisture is present on the fruit surface, and temperatures are in the 58° to 82°F range. Berries that have been damaged by insects, birds, machinery, etc. may become infected at any time after the fruit begins to ripen because the juice in the berry can provide the necessary water and nutrients for fungal growth.”

http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=514.00000

“Many fungal and bacterial organisms, of which Botrytis cinerea is the most important, can infect grapes and result in a bunch rot. B. cinerea has a large host range and grows and sporulates on most of them. The fungus overwinters and oversummers as black sclerotia on old cluster stems, on canes, and on mummified grapes. Spores spread by wind. Young, succulent shoots can be infected in spring, especially if injured by hail. Flower parts frequently are infected and can serve as a source of the fungus within the developing bunch. Wet weather favors infection and disease development, especially near harvest when canopies are dense and berries accumulate sugar. The fungus can quickly spread from berry to berry within ripening bunches and can develop readily on wounded or split berries.”