Mg and K against bunch stem necrosys

Bunch of Cabernet Sauvignon which clearly shows the desiccation septum (photo Bigot).

In the weeks before ripening, it is possible to note sporadically on the bunch some wiltings which may also affect major portions of grapes. These events have been observed on different varieties (Cabernet Sauvignon, Marzemino, Merlot, Sauvignon, etc.), and can be clearly separated into two typologies: bunch stem necrosys and berry shrivell or sugar accumulation disorder. Between the two physiological disorders, bunch stem necrosys has been most widely studied by several researchers; however, no effective solutions have been identified for its containment.

Symptoms and damages

The first symptoms of bunch stem necrosys can be observed soon after veraison, with necrotic lesions on the rachis or on some pedicels; hence the necrotic areas may expand to include a significant portion of bunch. The berries which are more distant from the necrotized rachis, are blocked both in development and metabolism, and slowly dehydrate. Analyzing the dehydrated berries, these highlight higher titratable acidity and lower sugar concentration. For this reason, at maturation, they should be removed. This operation results in a significant cost to the company when the damage begins to exceed 5% of the grapes.

Climate and nutrition

The factors that positively or negatively affect the extent of physiological disorders are mainly the weather course of the vintage and the nutritional balance.
Low temperatures during flowering or veraison are the key factors in triggering physiological disorders, but also changes in temperature may play an important role. Even rain during the ripening process has been associated with the onset of desiccation, both in Germany and in Australia.
As for mineral nutrition, there are inconsistent evidences, although many experimental trials have showed as a high ratio of potassium/ magnesium and/or calcium, and fertilization with magnesium and/or calcium could significantly reduce the problems in Europe. On the contrary, in trials carried out in California, fertilization with magnesium and calcium did not allow improving the situation, which even worsened after nitrogen applications.

Analyzed vineyards

Also in Friuli Venezia Giulia, as in other regions of Italy, some black grape varieties are particularly affected by bunch stem necrosys: besides Cabernet Sauvignon, there are also Refosco and Schioppettino.
In order to develop an effective intervention strategy against bunch stem necrosys, since 2009 some experiments of foliar fertilization on Refosco dal Peduncolo Rosso have been set up in in the area of Consorzio Tutela Friuli Colli Orientali and Ramandolo. In the hilly area, close to the border with Slovenia, four vineyards were identified, all grafted on SO4, which presented in several vintages problems of bunch stem necrosys.
Two vineyards (1 and 2) were characterized by a loam texture with a low supply of organic framework. The other two (3 and 4) showed macroscopic differences in texture: one vineyard with mostly clay and another vineyard characterized mainly by the presence of sand and stones. Chemical analysis of soils collected in the four vineyards showed normal average values of the main mineral elements.


In the three years from 2009 to 2011, a completely randomized experimental design has been set up for each of these vineyards, comparing 4 thesis of foliar fertilization with 3 repetitions (12 plots) for each vineyard (see table).
The four interventions were performed in the middle period of the growing season, the first placed in pre-flowering and the other at a distance of 12-14 days till the pre-veraison phonological stage..
During the growing season, leaf samples were collected at flowering, leaf petioles at veraison and grape berries at harvest, for the assessment of mineral elements content.
Moreover, close to harvest, a survey was carried out on 100 bunches/parcel, evaluating the severity and the incidence of bunch stem necrosis, , with attention in excluding similar symptoms due to sunburns or berry shrivell.


The damages caused by bunch stem necrosys are much more serious when the vineyard is located on alluvial soils with a low percentage of fine particles (silt and clay). In the area of Consorzio Friuli Colli Orientali, the vine-cultivated soils are characterized by the presence of de-carbonated marl, while in the areas in the North-East of Cividale there are alluvial soils and in these areas the main problems were normally ascertained. Moreover, the use of SO4 rootstock, characterized by lower vigor and low magnesium absorption, becomes another predisposing factor.
By looking at the effectiveness of different foliar treatments that have been tested, it is clear that no strategy has allowed a complete elimination of  the physiological disorders (chart 1).
The first thing which strikes the eye is the great variability among the different years, likely to be related mainly with the availability of soil water, which notoriously affects mobility and thus absorption of nutrients by the plant. The two vintages, 2009 and 2011, were characterized by a hot and dry summer especially near the harvest, and in this condition, potassium-based treatments (alone or mixed with Mg) have yielded the best results. On the contrary, in 2010, a vintage which proved to be cooler and highly rainy, the most satisfactory results were obtained with the use of magnesium. Potassium is a mineral element which is easily absorbed by colloids in the soil, and therefore, in the absence of circulating solution, it is poorly available for radical absorption. In rainy summers, potassium is instead absorbed in a special way compared to other nutrients, and plants easily run into problems due to magnesium deficiency.
While treatments with magnesium or potassium showed different behaviors depending on the vintage, the combination of the two elements has always allowed a reduction of the damage due to buch stem necrosys, in all vintages and in all vineyards, with an average efficacy of 32.3% in 2009, 37.3% in 2010 and 55.5% in 2011.
Since it is not possible to predict certainty the meteorological course of the vintage, based on the results obtained so far, preventive treatment with both mineral elements (magnesium and potassium) appears to be the only applicable agronomic remedy in order to reduce the incidence of bunch stem necrosys.


Hall et al. (2011) examined the sap flow produced in healthy grapes and attacked by bunch stem necrosys. They noted how phloem vessels suddenly ceased to carry sap when the symptoms appeared, causing thereafter tissues to die.


Important differences in damages from bunch stem necrosys have emerged in Trentino Alto Adige (Italy), in an experimental comparison of Marzemino grafted on different rootstocks. In 2006, as it was easily understood, the rootstock where the problem was more accentuated was Kober 5BB and SO4 (the difficulty in magnesium absorption is well known). Among the most performing rootstocks, there are 420A and 161.49 (Margoni et al., 2007).


FRANCESCO DEGANO – Consorzio Tutela Vini Friuli Colli Orientali e Ramandolo (Italy)
PAOLO SIVILOTTI – University of Nova Gorica (Slovenia)

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