The 2023 grape harvest in Italy experienced an unprecedented crisis, characterised by losses of more than 50% in some cases compared to the previous year. This dramatic drop in production is mainly attributable to the outbreak of the grapevine downy mildew, favoured by the excessive humidity caused by the heavy rainfall in May and June, which has caused, and continues to cause, serious damage in Italy’s wine-growing regions. The worst affected regions include Apulia, Sicily, Campania and Basilicata, as well as Veneto and Emilia Romagna. However, damage has also spread to Piemonte and Liguria.
We are therefore coming from a year in which the control of grapevine downy mildew was particularly complex throughout Italy, after about six years since 2018, which had been the last problematic year for downy mildew.
These premises only create great concern for grapevine downy mildew 2024: also because, experts say, we are even earlier than in the previous season, which was already early. In Emilia Romagna, for example, with the rains at the end of February, March and the beginning of April, the families of oospores have all finished their germination phase early, making many areas prone to infection. The early phenological phase of the vine is, as mentioned, more anticipated than last year, the earliest in recent history.
Before looking into the forecasts for the current year, let us analyse the various factors that have resulted in the failure to contain the disease in 2023. First of all, the climatic trend that favoured an early vegetative development, coinciding with abundant rainfall between April and May, which favoured primary infections that were particularly strong in the early stages of development, which were followed by secondary infections caused by frequent rainfall in June and July.
However, despite these adverse conditions, it would have been possible to control downy mildew using the plant protection products currently available.
Unfortunately, there were some contingent situations in which downy mildew spread, creating serious problems. This critical situation occurred, in some areas, due to the impossibility of entering in the field to carry out treatments as too much water and humidity in the soil did not allow for the movement of the sprayers.
However, it is important not to underestimate possible carelessness of producers, who in some areas delayed the start of treatments and excessively lengthened the intervals between treatments at particular times.
Also very problematic was the slowdown in the defence against downy mildew from the beginning of July, which, especially in the wettest areas and in the presence of active outbreaks, favoured a very consistent development of downy mildew on the females, causing significant damage in the pre-harvest phase.
Careful defence has, however, always managed to keep the situation under control. The problems, as we have said, came when defence started late, was neglected or too long intervals between treatments were adopted and plant protection products were used at times that were not consistent with their functional characteristics.
One factor that has pushed in this direction is undoubtedly climate change, which is interfering more and more significantly with the spread of plant diseases, and which in 2023 contributed to the earlier occurrence of downy mildew than in previous decades.
In this scenario, forecasting models on the development of downy mildew were put to the test, which nevertheless confirmed good reliability and proved to be very useful for placing the first interventions. In particular, it was confirmed that the ‘three tens’ rule (Fig. 1), an extraordinary intuition for understanding the development of downy mildew, is now substantially outdated and, regardless of vegetative development, primary infections very often appeared as early as the end of April with limited vegetative development, adequate wetting and temperatures above 10 degrees centigrade.
The ‘three tens’ rule
Primary infections begin in spring when certain microclimatic conditions, known as the ‘three tens’ rule, occur simultaneously:
– The 24-hour average temperature must be around 10 C;
– The average length of the young shoots must be at least 10 cm (receptivity of the stomata);
– Rainfall over the last 1-2 days must have been at least 10 mm;
Source: arsacweb.it
In particular, the models proved to be fundamental for starting the defence in 2023, when the early vegetative awakening of the crop was accompanied by a very marked anticipation of primary infections, which appeared as early as mid-April. More precise and punctual could be the use of models on a farm scale with the help of local weather stations, and the use of models for the management of phytosanitary defence on a territorial scale is still very useful. This has been confirmed by the last few years in which defence strategies are now based exclusively on preventive measures, guided by weather forecasts.
As far as 2024 is concerned, we refer to the words of the Agrea test centre researcher, Gabriele Posenato, who believes that the severe infections of 2023 may not adversely affect the development of downy mildew in 2024, which will obviously depend on the rainfall conditions in the coming months.
In terms of strategy, he considers that it is very important to choose and apply plant protection products at times when they can best perform their activities. Indicatively, he suggests to schedule the applications as specified in the table below.
It is always useful to remember the phenological stages in which the plant is most prone to infection:
Fig. 5 Flower cluster initiation
Fig. 6 Pre-flowering
Fig 7: Fruit-set
Fig 8 Cluster pre-closure
Source: arsacweb.it
In the end, everything suggests that, in the fight against downy mildew, the farmer’s experience together with the monitoring of plot microclimatic data may be the most effective combination for predicting the occurrence of this disease and taking action in time.
There are now a number of solutions on the market which, through IoT sensors installed in the vineyard, aim to support the winegrower in agronomic decisions, not replacing his own experience and that of agronomists but only providing information and processing of data acquired directly on the field.
This is a battle we can win with some care and attention!
Sources:
https://agronotizie.imagelinenetwork.com/