Balanced soils to counter the death of kiwifruit

The Kiwifruit Vine Decline Syndrome (KVDS), better known as kiwi death, is still not fully understood and in Italy was observed for the first time in 2012 in Veneto, from where it spread to the other cultivation areas, which are mainly concentrated in five regions. In descending order of surface area, Lazio, Piedmont, Emilia-Romagna, Calabria and Veneto cultivate Actinidia in our country, for a total of about 24,000 hectares (2021 data), of which about 77% are cultivated with green pulp varieties, 22% with yellow pulp varieties and 1% with red pulp varieties. In the world ranking of kiwi producing countries, in 2021 Italy – which produced about 305,000 tons of Actinidia, a sharp decline compared to previous years (CSO Italy data) – ranked at third place, after China and New Zealand.

It is estimated that today in Italy, about 26% of the total area cultivated with kiwi is affected by this death. All cultivars can be affected, regardless of the color of the pulp. In addition to having a direct negative effect on yields and therefore on the PLV, the spread of this death leads to increases in the management costs of the orchard and the trend by fruit growers to employ resources in an attempt to contain the problem; similar to what happened in previous years with the Actinidia bacterial cancer, cause by the bacterium Pseudomonas syringae pv. actinidiae (PSA).

The death of kiwifruit manifests with the wilting of the shoots and decay of the crown, followed by a reduction in the size of the fruits and failed ripening, with obvious drop in production and later the death of the plant, which can sometimes occur shortly after the onset of symptoms.

This is because the problem lies in the root system and often the plant manifests the symptoms in its epigeal portion when the syndrome is already at an advanced stage. In fact, it is estimated that Actinidia can continue to vegetate and produce in an almost normal way even with a 65% functional impairment of the roots. In the root system of the affected plants, bark detachment, loss of fuzz, widespread rot and occlusion of the vessels can be noticed, resulting in the impossibility for the plant to feed properly.

Subjected to intense research by the Universities of Verona, Udine and Bologna,  KVDS has shown to manifest itself more frequently in the presence of specific abiotic factors. Among these, the physical characteristics of the soil, in particular its porosity, and the management of irrigation are of fundamental importance.

Dying occurs in higher percentages in the presence of water stagnation, therefore, the combination of widespread soil compaction in plots growing Actinidia, aided by the increased steps with agricultural vehicles carried out in recent years for defense interventions against PSA, and irrigation that is not always rational, has promoted the dissemination of KVDS. Kiwifruit is very demanding in terms of water and this sometimes leads the fruit grower to exceed watering volumes, especially in the case of flowing irrigation. Waterlogging and soil saturation reduce the amount of oxygen available to the roots and hinder their growth – which in kiwifruit has two peaks, one in spring and one in autumn – and proper functioning. More specifically, the root damage that occurs during the autumn season compromises the plant recovery in the following year, aggravating any critical spring situations. In addition to irrigation, climate change has contributed in emphasizing this unfavourable outcome in recent years, with the irregular distribution of rainfall which negatively affects the structure of the land.

A higher incidence of death was also observed in the presence of increased soil salinity and accumulation of heavy metals, including copper used to control PSA.

The microbial composition of the rhizosphere has also proven to be a very important factor in determining whether or not the onset of kiwi death occurs.

At the moment, a series of fungi typical of water-saturated environments have been identified in the affected plots, already known as being responsible for damaging the roots and collar of kiwifruit, among the different species of the genera Phytopythium and Phytophtora, however, anaerobic bacteria of the genus Clostridium are also involved.  Metagenomics studies on the soils of these plots have clearly demonstrated important changes in the composition of the microbiome, although a close link with the pathogens believed to be responsible for the root symptoms has not yet been proven.

While waiting for breeding to select varieties and rootstocks that can withstand the KVDS or at least tolerate it while guaranteeing adequate productions, in the short to medium term the containment strategies are based mainly on agronomic technique.

Rational irrigation management, based on the actual water needs of the plant, must be combined with the careful management of the land, both in terms of placement – with the construction of ridges that counteract water stagnation – but above all with actions aimed at maintaining and/or increasing the stable organic matter in the ground. This is not only to promote the porosity of the soil, but also to create favourable conditions in the rhizosphere for the proliferation of useful microorganisms, able both to promote the nutrition of the plant, and to occupy the ecological niche, to the detriment of microorganisms involved in root rot.

Considering the importance of optimal conditions of porosity and oxygenation of the soil in counteracting the establishment of KVDS in kiwi crops, whether organic or integrated, there are various organic and organo-mineral fertilizers offered by Agribios that may be of absolute interest in Actinidia nutrition strategy.

However, we believe it’s useful to focus mainly on products from the Terrapiù line, inoculated with useful microorganisms.

AGRI BIO AKTIV (pellets) is a soil activator made of humidified organic matter, bacterial and fungal inoculums. AGRI BIO AKTIV is a soil regenerator that, in addition to providing organic matter and nutrients, promotes and directs the proliferation in the soil of microorganisms useful for the development of plants. Furthermore, its particular microbial composition, rich in Trichoderma harzianum and viride and rhizosphere bacteria, limits the development of pathogens responsible for certain diseases of the root system such as Fusarium, Pythium, Armillaria, Sclerotinia, Phytophtora and Rhizoctonia.

With a similar action, BIO AKTIVEG (pellets) is characterized by a 100% vegetable organic matrix, making it suitable for all companies subject to limitations concerning fertilization with animal derived products.

Microtech Start

MICROTECH START is a liquid organic fertilizer, 100% vegetable-based, recommended at the two peaks (spring and autumn) of kiwifruit root growth. Its special composition, the result of scientific research and company know-how, enables it to provide plants with the key elements of nutrition (nitrogen, phosphorus and potassium) as well as organic carbon, enzymes, sugars, levorotatory amino acids, peptones, mycorrhizae and rhizosphere bacteria (PGPR, nitrogen fixators, etc.). The interaction between the organic matter and a specific microbial pool, creates an extraordinary synergy between all the components of MICROTECH START, greatly benefitting the soil and the crop planted.