Foliar pH was monitored over a five-year period to ascertain whether and how a significant trend could be detected in grapevines. When the results showed a diminutive parabolic pattern, we were astonished and searched for an explanation. We then paired our observations on the field with sunspots, and something began to become clear. Moreover, NIR-SCiO spectra, coupled with pH measurements, allowed us to decipher the correlated changes in the gross composition of the leaves. Finally, a retrospection of historic outbreaks, paired with the sunspots, corroborated by bibliographic results, began to open new scenarios.

During the five years (Figure 1), the average pH decreased almost regularly from 3.73 in 2015 to 3.15 in 2017, then appeared stable, according to the negative linear and the positive quadratic components (Table 1). However, the pattern was different for the six vines (significant interaction). A general common trend was verified in two vines (Cabernet and Barbera). A shift in height occurred in Pinot in 2018. After a minimum was reached in Shiraz and Nebiolo in 2017, the pH began to increase. The pH values in Grenache were lower in 2016 and 2019.

The correlation of the foliar pH with the NIR reflectance spectra ranged from -0.34 to -0.59 (Figure 2) and the composition of the leaves varied accordingly (Table 2). In a condition of low pH, leaves tend to contain more water, and to raise the crop maturity index, NDF digestible, protein, ADF, cellulose, hemicellulose, lignin, energy and crude fiber. In a condition of high pH, leaves tend to lose the lipid, ash, NDF digestible, total digestibility and free sugar contents. In short, a lowering of the pH, in response to a few sunspots, induces the plant to privilege the wall at the expense of the vacuole and the cytoplasm. Moreover, a higher maturity level tends to favor a rapid and amplified reproduction phase.

Two parallel parabolic traces described a conjoint descent of the sunspots and the foliar pH over the years (Figure 3) A Pearson correlation of 0.95 (P 0.01) linked the two parameters. This is a new finding, but it does not seem to be based merely on statistics, for the reason discussed thereafter.

We here present a data mapping of the outbreaks that were reviewed by plant pathologists (Table 3), from which it appears that several outbreaks (Figure 4) took place during some periods of low sunspot occurrences. Considering a 50% possibility of falling below the sunspot average of the involved solar cycle, twenty-two of the twenty-five referenced cases of outbreaks were below the average (P 0.0001) and often at minimal values, as can be seen from the average 49% decrease.

As far as insect pests are concerned, after assessing some of the Phylloxeravastatrix outbreak statistics in the grapevines, the relationships appeared positive (Table 4): only eight out of thirteen historic cases (61%) were below the mean value of the cycle and the sunspots were significantly correlated (r 0.78; P 0.001).

However, a clear confirmation of the pattern shown in Figure 3 was derived from long-term experiments and scientific studies on the forest moth pest species. Selas et al. 15, from 1970 to 2004, observed a clear inverse correlation that opposed the presence of sunspots to the outbreaks: r -0.92 in the autumnal moth (Epirritaautumnata) in Betula and r -0.78 on Eilemalurideola from a lowland mixed forest.

A further confirmation of a possible, non-random cyclical correlation between sunspots and epidemiology can be derived from the accurate monitoring of the fire blight pest in Trentino 16. In Figure 5, it is possible to observe that the frequencies were relatively stable and the sunspots were unrelated in the middle of two full solar cycles, in the 1999-2011 interval. However, in the interval 1999-2003 and after the maximum outbreak occurred in 2011, problaby due to extraordinary temperatures and heavy rainfall, the two lines show strong opposite trends (r -0.88; P 0.049 and -0.80; P 0.0172, respectively).

The only multispecies study concerning the functionality of foliar pH 6 hypothesized that the tissue pH itself is closely controlled by a given species, because of its direct or indirect functions in the plant. For instance, a low pH has been found to correspond to poor digestibility, and may therefore act as an anti-herbivore defence in subarctic flora.

In a previous paper 8, we outlined the pivotal role of foliar pH in changing the composition of the leaves in conventional or mycorrhizal Sorghum. The results of the present work have confirmed the positive co-variation of pH with the dry matter, sugars and NDF as well as the negative co-variation of the ADF, hemicellulose, lignin, gross energy and crude fiber. Some constituents now appear different, namely the lipids (now positive) and the cellulose (now negative). However, the general pattern of leaf acidification manifested by high hydration and reiforcement of the wall compartment shown in the Sorghum study 8 has here been confirmed for grapevines. In general, although more resistant walls are favorable, a greater turgidity of leaves is negative for the defense against parasites.

As far as the negative correlation of the sunspots with mud insect development is concerned 15, this unexplained fact was deemed to be due to a differential UV-B irradiation in a high sunspot phase which, in turn, led the plants to lower their chemical protectants. A low sunspot activity leads to a thinner ozone layer and thus a higher surface ultraviolet (UV)-B radiation. As winter moth larvae prefer leaves subjected to enhanced UV-B radiation, the Authors argued that the statistical relationship between sunspots and moths could be substantiated by the metabolic costs of producing UV-B-protective pigments during periods of low sunspot activity, which reduce tree resistance to herbivores, and thus increase the survival of moth larvae. The sun emits light, heat and UV radiation. The UV region covers the 100–400 nm wavelength range and is divided into three bands: UV-A (315–400 nm) UV-B (280–315 nm) UV-C (100–280 nm). As sunlight passes through the atmosphere, all the UV-C, and approximately 90% of UV-B radiation, are absorbed by ozone, water vapor, oxygen and carbon dioxide. UV-A radiation is less affected by the atmosphere. Therefore, the UV radiation that reaches the Earth’s surface is mostly composed of UV-A with a small UV-B component. The global solar UV Index (UVI), graded from 1 to 11⁺, is a measure of the intensity of UV radiation on the Earth’s surface and it is relevant because of its effects on human skin 17. WHO who.int/uv has developed reliable predictions of the consequences on human health and on the environment due to changes in UV exposure as a result of stratospheric ozone depletion. Maps of the daily UV irradiation at the soil level depend to a great extent on the season and latitudes. Nordic countries (Norway, Sweden and Finland) tend to receive higher levels of UV irradiation than Germany or France in late spring and early summer.

According to the examined outbreak cases, a statistic hypothesis of an association with sunspots cannot always be rejected. However, only a few serious long-term studies are available to corroborate a general theory on such an argument. Taking inspiration from the mud-insect studies, a solar influence could be envisaged on the quantity of the UV-B reaching the plants, and the ozone layer is involved to a great extent. In short, if the sunspot/pH relationship that has emerged for the first time in the present work could be back-projected correctly, the plants would be induced to be in a acidic status, when and where the parasites could have manifested their maxima attacks, in true historical events not mitigated by pesticides. An increase in the circulating acidity of the plants has here been hypothesized to play a role in diminishing the general tendency to block parasite attacks. According to the present pH model hypothesis, the evolution of the plants might have been driven in primis by some of the photo-energetic assets and in secundis by the parasitisms being overdominated by the fungi, which are well adapted to growing in the acidic environment of plants. Cornelissen et al.18 showed a variety of acidic levels that characterize the green foliar world, and this foliar pH is totally independent of the soil pH. In a previous study 7, we confirmed the cornucopia of acidic substances that determine a low pH in plants, and highlighted an inverse correlation between the number of fungicide treatments and the foliar pH (R² 0.9). Moreover, the foliar pH was inversely correlated to water stressed conditions, and  with the temperature, which is, overall, the main abiotic factor in agriculture.

Volpato et al.19, using a pH mycorrhizal model, outlined that some maize varieties did not respond by lowering their foliar pH after inoculation, i.e. were not responsive to symbiosis, as they were oriented toward a commensalism or to parasitism. Thus, the change in the energetic status is the key to understanding the critical sustainability of plants over the short horizon of a sunspot cycle, where critical points (<7 sunspots) are observed in 1 out of 5.66 years, i.e. in 18% of the cases or two per cycle 9. The preceding discussion was focussed on the foliar pH responses to the diminishing sunspots in this 24^th cycle. This is a physioclimatic response to a reduction in sunspots. But what the “trojan horses” that threaten plants are could remain unknown.