COS 162-6 - How do shade, nitrogen and fungicide interact to shape plant-associated fungal communities?

Abstract: Light and nitrogen availability shape photosynthetic rate, influencing the amount of energy plants can allocate to their own growth, defense and reproduction, as well as the availability of energy for their herbivores and mutualists. Plants in shadier environments may have less energy available for defense than their counterparts in sun, while experiencing higher humidity that favors many fungal pathogens. As a result, plants could invest less in protective mutualisms. Alternatively, they could slice the energy pie differently, allocating less to other functions in order to maintain protective mutualisms. To better understand how plant resource availability influences communities of associated fungal pathogens and mutualists, we established a field experiment in 2022, in which we manipulated light and nitrogen availability, and also used a fungicide treatment to extrinsically reduce fungal diversity, in a 2×2×2 factorial design in a 1 Ha site divided into 104 plots of 36 coffee plants each, taking advantage of coffee’s ability to grow under a wide range of light availability.

There was no difference between high(60%) and low(35%) shade plots in incidence or severity of disease two months after shade establishment, with symptoms of an anthracnose by Colletotrichum frequent. Fungicide use had an immediate negative effect on mutualistic fungi, resulting in lower root colonization by arbuscular mycorrhizal (AM) fungi, lower hyphal length in the soil, and lower sporulation by several species of AM fungi one month after the first fungicide application. Fungicide application did not affect severity of disease, but disease incidence was lower for coffee leaves in plots treated with fungicides. Coffee yield in the first year of fruiting was influenced by shade and fungicide application but not by nitrogen fertilization. Plants in fungicide plots produced more coffee fruits than plants in plots without fungicides, while plants in higher shade produced heavier fruits than those in lower shade. We are currently characterizing root and leaf fungal communities using ITS2 metabarcoding. Higher shade environments may not result in increased disease, despite a more favorable physical environment, if plants allocate resources toward protective mutualisms and plant defense over other functions. Short-term escape from disease via fungicide application may have long-term disadvantages if fungicides suppress key mutualisms.