Injury related bud necrosis. The inflorescence appears to be rotting.

Plant Growth Regulator (PGR) compounds — Presumably, PMBN is mediated by PGRs that respond to the physiology of the plant and the environment, by signaling certain buds to begin necrosis. The precise mechanism is not known.

Too much gibberellin can increase PMBN, although not all studies have found a correlation. Gibberellin applied early (bloom time) has a stronger effect than when applied later (sizing sprays), and more distal buds are affectedmore than those near the base.5

Several growth retardants can decrease PMBN. Some of these growth retardants act by blocking gibberellin activity, leading some researchers to conclude that gibberellin is the principle PGR signaling buds to die.5

The effect of cytokinins on PMBN is not known. However, some limited observations suggest that post-bloom kelp sprays, which have cytokinins, can increase fruitfulness in Thompsons the following year. The post-bloom period iswhen PMBNoccurs, so the cytokinins may be reducing PMBN. This period is probably too late for the cytokinins to be influencing inflorescence.

Water — Too much water has been shown to increase PMBN. This is attributed to extra vegetative growth using up carbohydrates. I have not found any studies linking PMBN to water deficit.

Temperature —I have not found any studies linking PMBN to high or low temperatures. Very low temperatures are not likely to occur in the May-June period when most PMBN occurs. High temperatures could possibly cause injury-related bud necrosis (see photo, page 16).

Excess nitrogen — One would expect that if extra nitrogen increased shoot vigor, it might also increase PMBN. One study found that vineyards with excessivemanure hadmore dead buds.5

Not mineral deficiency — Several studies found no correlation between PMBN and deficiency of minerals, including nitrogen, phosphorus, potassium, calcium, magnesium, and boron.5

In times of carbohydrate shortage, buds seem to be a low plant priority, and PMBN can be expected to increase. For example, vineyards that are heavily cropped one year often show reduced fruitfulness the following year. Berry growth pulls carbohydrates away from the buds, and the plant may respond by sacrificing buds through PMBN. Indeed, the period of most rapid berry growth seems to coincide with the majority of PMBN.

Conditions that reduce available carbohydrates, including shading and excessive shoot vigor, also increase PMBN.

Shoot vigor—Excessive vegetative growth in fast-growing canes soaks up carbohydrates, so they are not available to the buds. Several studies have found more PMBN in more vigorous shoots (thicker and with longer internodes), but the correlation is not strong and has not been found in all studies.5

Vigorous shoots tend to have lower carbohydrate levels, because the carbohydrates are flowing to new growth. Vegetation also increases shading, another factor that increases PMBN. Farming practices that discourage excessive growth might improve fruitfulness. However, if these same practices decrease carbohydrate production, the net effect on fruitfulness might be zero or negative.

Cutting canes of Riesling vines increased PMBN, but only in the buds close to the cut. The shootswere tipped 40 days after bud break at approximately 18 nodes. PMBN increased in buds 13 to 20. The PMBN was attributed to growth of lateral shoots which competed for carbohydrates.5

Shading — Shading is an important contributor to PMBN.5 During the summer months, shading is typically from vegetative growth. Buds in the inner, darker parts of vines, under heavy foliage, are more likely to suffer PMBN than those in better-exposed parts.

Unfortunately, the inside buds include most of those that will be selected by pruning for the following year. Leafing, hedging, and summer pruning will increase light into the canopy and might reduce PMBN.

Several studies have found that shoot thinning decreased PMBN, which was attributed to less shading. One study found the opposite: more PMBN after thinning. This was possibly due to increased vigor of the remaining shoots.5

Some studies indicate that only long periods of shade increase PMBN (40 days compared to less than 21). If this is true, then clouds are not likely to be important in California, where long periods of cloudyweather are unknown during summer months.5