Mold pressures & cold weather
Tight clustered fruit like the Pinot Noir pictured here has plenty of mold spores around from the late rains we had last Spring. Wet humid weather allows the molds to germinate, and it essentially destroys the cluster from the inside out. This year we've had our fair share of humid cool foggy weather - and then some - which is driving this phenomena forward. Areas of the affected clusters start to shrivel and the mold then infects more berries right next to those originally hit - spreading through the center of the cluster & making it worthless.(note the decent sized berries @ the top, and how the progression from "good" to "bad" occurs as you look towards the middle of the cluster...)
I'll be dropping this compromised fruit to the ground so it's not included with the good fruit being picked. Otherwise it would just spike the sugars upward maybe causing the fermentation to stick, give the wine an "off" moldy note, and inhibit the yeast fermentation further by competing for nutrients - which again could cause off aromas and stick the fermentation.
Not what you're looking for in a good wine...especially with the forecast calling for very cool weather, and possibly light rain, when you're still a week & a half away from harvesting this block.
Reminds me of the mess of both '95 & '98 harvests with the amount of fruit still hanging on the vines in Carneros and Russian River areas (Tom - read that as "Sonoma Coast"), and the weather starting to turn.
Ahhhh, the bucolic life of a farmer...how idyllic indeed!