Various fungi and bacteria degrade dead plants, however, these microorganisms may also affect the xylem of living plants. The Esca syndrome of vine plants and the sudden death syndrome of soybean are two examples of diseases affecting agriculturally important plants leading to withering of the plants and crop failures. These diseases are caused by an imbalance of microorganisms degrading lignin and other components of the xylem, which itself is triggered by stress, like heat and dry spell.
At the same time, microorganisms like fungi and bacteria are not harmful per se, they are even known to be important for plant growth. Thus, a general and undirected application of pesticides might have unfavourable results.
In human medicine, the controlled and site specific delivery of drugs is a hot topic and this smart approach can be used for plants as well. A novel nanocarrier suitable for drug delivery in plants is made from chemically modified lignin: By esterification of lignin with methacrylic anhydride a polymerisable but bio-degradable macromonomer is prepared, which is polymerised in a miniemulsion. The miniemulsion polymerisation gives access to nanoparticles of controlled size and morphology and allows easy addition of 5 to 35 % different pesticides into the lignin matrix (see Fig. 1).
The nanocarriers loaded with pesticides can be injected to the xylem of plants. If the plant is weakened, microorganisms infest the xylem and degrade lignin. Untreated, this leads to apoplectic wither of sprout and leafs. However, if the lignin nanocarriers have been injected to the xylem, the microorganisms degrade these too, releasing the pesticide.
Figure 1: Illustration of the preparation of lignin nanocarriers with variable morphology by miniemulsion polymerisation and TEM image of lignin nanocarriers.
Figure 2: Result of field study: severeness of Esca syndrome decreased drastically on 80 monitored vine plants due to treatment with fungicide loaded lignin nanocarriers.
Thus, the pesticide is released only in case of infestation making the lignin nanocarrier act like a vaccination. Furthermore, by this strategy the amounts of pesticides used is decreased to less than 3 % of the typical amount used in spraying. The efficacy of the lignin nanocarriers to fend the Esca syndrome has been tested successfully in a field study with 80 vine plants. Half of these had been treated with with a single dose of lignin nanocarriers loaded with 20 wt% fungicide in 2014 (<10 mg pyraclostgrobin per plant injected in the trunk) while the other half has not been treated. The first group showed significant less severe Esca symptoms compared to the second one (see Fig. 2). After three years without further treatment, the difference in severeness has even increased. No pyraclostgrobin was detected in the grapes of the plants, presumably due to the size (200–500 nm) of the nanoparticles.