"Phytopathological strolls" in the dual context of COVID-19 lockdown and IYPH2020: Transforming constraints into an opportunity for public education about plant pathogens

The experience presented here relates to 2020, a particularly timely year for plant disease-related communication (International Year of Plant Health, IYPH2020), but also a unique year because of the COVID-19 pandemic. Our goal was to illustrate the diversity and beauty of fungal plant pathogens through a naturalist approach that could be followed by any amateur. We achieved this end through "phytopathological strolls", in which we observed and determined the origin of symptoms on diseased plants found in our garden, in the local streets, and in nearby open spaces, and shared this matter with a broad public. The lockdown imposed in France created an additional motivation to take up the challenge, and to involve our children, even under strong constraints such as movement restrictions. We observed and described fungal pathogens through hundreds of photographs, shared our findings with a large audience on Twitter, and received feedback. The material used was deliberately simple and transportable: a digital reflex camera, an old microscope, a mobile phone, some books, and an internet connection. Between 17 March 2020 and 20 June 2021 we found 196 plant pathogens, including 97 rusts, 27 powdery mildews, and 28 septoria-like diseases. We discuss here the importance of promoting searches for plant pathogens, their description and conservation, through a combination of classical approaches and digital tools in tune with the times, such as Twitter, by treating pathogen identification like a detective game and, more surprisingly, by making use of the addictive nature of collection approaches, drawing a parallel with Pokémon GO.

Possible control of acute outbreaks of a marine fungal pathogen by nominally herbivorous tropical reef fish

Primary producers in terrestrial and marine systems can be affected by fungal pathogens threatening the provision of critical ecosystem services. Crustose coralline algae (CCA) are ecologically important members of tropical reef systems and are impacted by coralline fungal disease (CFD) which manifests as overgrowth of the CCA crust by fungal lesions causing partial to complete mortality of the CCA host. No natural controls for CFD have been identified, but nominally herbivorous fish could play a role by consuming pathogenic fungi. We documented preferential grazing on fungal lesions by adults of six common reef-dwelling species of herbivorous Acanthuridae and Labridae, (surgeonfish and parrotfish) which collectively demonstrated an ~ 80-fold higher grazing rate on fungal lesions relative to their proportionate benthic coverage, and a preference for lesions over other palatable substrata (e.g. live scleractinian coral, CCA, or algae). Furthermore, we recorded a ~ 600% increase in live CFD lesion size over an approximately 2-week period when grazing by herbivorous fish was experimentally excluded suggesting that herbivorous reef fish could control CFD progression by directly reducing biomass of the fungal pathogen. Removal rates may be sufficient to allow CCA to recover from infection and explain historically observed natural waning behaviour after an outbreak. Thus, in addition to their well-known role as determinants of macroalgal overgrowth of reefs, herbivorous fish could thus also be important in control of diseases affecting crustose coralline algae that stabilize the foundation of coral reef substrata.

Herbivory meets fungivory: insect herbivores feed on plant pathogenic fungi for their own benefit

Plants are regularly colonised by fungi and bacteria, but plant-inhabiting microbes are rarely considered in studies on plant-herbivore interactions. Here we show that young gypsy moth (Lymantria dispar) caterpillars prefer to feed on black poplar (Populus nigra) foliage infected by the rust fungus Melampsora larici-populina instead of uninfected control foliage, and selectively consume fungal spores. This consumption, also observed in a related lepidopteran species, is stimulated by the sugar alcohol mannitol, found in much higher concentration in fungal tissue and infected leaves than uninfected plant foliage. Gypsy moth larvae developed more rapidly on rust-infected leaves, which cannot be attributed to mannitol but rather to greater levels of total nitrogen, essential amino acids and B vitamins in fungal tissue and fungus-infected leaves. Herbivore consumption of fungi and other microbes may be much more widespread than commonly believed with important consequences for the ecology and evolution of plant-herbivore interactions.

Gall induction may benefit host plant: a case of a gall wasp and eucalyptus tree

Gall-inducing insects display intimate interactions with their host plants, usually described as parasitic relationships; the galls seem to favor the galler alone. We report on a case in which the presence of the galls induced by Leptocybe invasa Fisher & LaSalle (Hymenoptera; Eulophidae) benefit its host plant, the river red gum Eucalyptus camaldulensis Dehnh. Field observations showed that E. camaldulensis plants infected by this gall wasp were less susceptible to cold injury than neighboring conspecific plants without galls. In the laboratory, frost resistance was compared between galled and non-galled plants which were both divided into two subgroups: cold-acclimated plants and plants that were non-acclimated. Galled plants displayed higher frost resistance than the non-galled ones, and the differences were higher in non-acclimated plants compared with acclimated ones. Physiological changes in host plant were determined by chemical analyses of chlorophylls, proteins, soluble sugars and anthocyanin contents. The results showed higher values of all physiological parameters in the galled plants, supporting the hypothesis that the presence of the gall wasp induces physiological changes on the plant foliage, which may in turn increase plant defense mechanisms against cold. Therefore, the toll of galling by the herbivore may pay off by the host plant acquiring increased frost resistance. This work provides evidence for physiological changes induced by a herbivore which might have a positive indirect effect on the host plant, promoting frost resistance such as cold acclimation.

The effect of an insect herbivore and a rust fungus individually, and combined in sequence, on the growth of two Rumex species

The chrysomelid beetle Gastrophysa viridula and the rust fungus Urmnyces rumicis both occur on leaves of Rumex crispus and R. obtusifolius. We investigated the effect of beetle grazing or rust infection individually and when combined in sequence on the growth of their hosts in the field. Singly, beetle or rust reduced leaf area and plant biomass; the effect was greater on R. crispus, and rust caused greater damage than the beetle. Beetle grazing with subsequent rust infection caused damage no greater than that caused by rust alone, although on R. obtusifolius damage was greater than that from beetle grazing alone. Rust infection of R. obtusifolius with subsequent beetle grazing produced damage similar to that from other treatments; involving rust infection. In R. crispus this treatment produced the greatest reduction in biomass, The reductions in root and total plant weight from rust infection with subsequent beetle grazing were accurately predicted by a model including the damage produced by beetle and rust alone and the length of time each was present on the plant. This model also predicted accurately the damage to R. obtusifolius from the beetle followed by rust treatment, but over-estimated by up to 40% the damage to R. crispus. This can be explained mainly by an inhibition of rust infection by beetle grazing.