Ultraviolet-C (UV-C) radiation is efficient in reducing the development of diseases in many species, including strawberry (Fragaria × ananassa). Several studies suggest that UV-C radiation is effective not only because of its disinfecting effect but also because it may stimulate plant defenses. In this study, the effect of preharvest UV-C radiation applied during strawberry cultivation on plant growth, fruit quality, and susceptibility to major fungal diseases such as gray mold, powdery mildew, and soft rot was evaluated. UV-C treatments had an impact on flowering initiation and fruit development. Flowering occurred earlier for UV-C-treated plants than for nontreated plants. At harvest, a larger amount of fruit was produced by treated plants despite their slight decrease in leaf area. UV-C treatment did not improve strawberry shelf life but did not alter the physical integrity of strawberry fruit. Natural infection of leaves to powdery mildew and of fruit to Rhizopus spp. strongly decreased in response to UV-C treatment.

Systemic acquired resistance is a powerful mechanism, based on the salicylic acid (SA) signaling pathway, which allows plants to resist to a wide range of pathogens. High SA, moreover, plays a key role in plant tolerance to abiotic stress. It seems, therefore, desirable to supply analogs of SA or stimulate the production of endogenous SA. Unfortunately, the chemical substances or physical means used for this effect often display a variable efficacy. After providing a review of them, we defend three major ideas: (i) plant resistance inducers (PRIs) must be combined for higher efficacy, notably for exploiting synergic effects between the SA and other signaling pathways, (ii) disease pressure can be reduced by exploiting the fungicidal properties displayed by some PRIs, (iii) biostimulants and crop management techniques should be used to ensure that plants have the resources they need to synthesize the compounds and structures required for efficient and lasting resistance. Some PRIs could also be used for their biostimulant effects in stress conditions. It could be concluded that holistic approaches which jointly address the issues of defense and tolerance stimulation, disease pressure and resource availability in plants are the ones that will allow for substantial reduction in fungicide use without sacrificing crop performance.

Using detached leaves, UV-C light in the form of 1-s flashes has recently been shown to stimulate defenses of several plants against different pathogens better than 1-min exposures under greenhouse conditions. In the present work, the pathological tests were conducted using undetached leaves under greenhouse and vineyard conditions. In a first trial, two flashes of UV-C light were applied to plants of Vitis vinifera L. ‘Chardonnay’ grown under greenhouse conditions, at an interval of 10 days. Plants were inoculated with Erysiphe necator 2 days after the last light treatment. After 18 days of inoculation, the symptom severity on leaves was reduced by 60% when compared with the untreated control. In a second trial, flashes of UV-C light were applied to grapevine Chardonnay plants under field conditions in the southeast of France every 10 days from 18 April until 10 July 2019. The symptom severity resulting from natural contaminations by E. necator was reduced by 42% in leaves on 4 July 2019 and by 65% in clusters on 25 July 2019. In a third trial, we observed that UV-C light did not have any effect on net photosynthesis, maximal net photosynthesis, dark respiration, maximal quantum efficiency of photosystem II, the performance index of Strasser, and, generally, any parameter derived from induction curves of maximal chlorophyll fluorescence. It was concluded that flashes of UV-C light have true potential for stimulating plant defenses against E. necator under vineyard conditions and, therefore, help in reducing fungicide use.

Pepper is known to be a recalcitrant species to genetic transformation via Agrobacterium tumefaciens. A. rhizogenes-mediated transformation offers an alternative and rapid possibility to study gene functions in roots. In our study, we developed a new and efficient system for A. rhizogenes transformation of the cultivated species Capsicum annuum. Hypocotyls and foliar organs (true leaves and cotyledons) of Yolo Wonder (YW) and Criollo de Morelos 334 (CM334) pepper cultivars were inoculated with the two constructs pBIN-gus and pHKN29-gfp of A. rhizogenes strain A4RS. Foliar explants of both pepper genotypes infected by A4RS-pBIN-gus or A4RS-pHKN29-gfp produced transformed roots. Optimal results were obtained using the combination of the foliar explants with A4RS-pHKN29-gfp. 20.5% of YW foliar explants and 14.6% of CM334 foliar explants inoculated with A4RS-pHKN29-gfp produced at least one root expressing uniform green fluorescent protein. We confirmed by polymerase chain reaction the presence of the rolB and gfp genes in the co-transformed roots ensuring that they integrated both the T-DNA from the Ri plasmid and the reporter gene. We also demonstrated that co-transformed roots of YW and CM334 displayed the same resistance response to Phytophthora capsici than the corresponding untransformed roots. Our novel procedure to produce C. annuum hairy roots will thus support the functional analysis of potential resistance genes involved in pepper P. capsici interaction.