Think again before you compost that

Spring has arrived, gardens are planted, and now, we eagerly await the harvest of fruits and vegetables. This spring awakening brings not only new plant life, but fungi also come out to feast. Phytophthora root rot is a common fungal disease in plants, infecting over 250 plant genera including peppers, tomatoes, berries, and eggplants. At least a hundred species of fungi are responsible for phytophthora root rot. Chemical efforts to treat phytophthora root rot have been ineffective to control disease and have mostly been banned. So what is a gardener or farmer to do?

Utilize the power of microbial decay, of course! Despite the stink, compost has become all the rage. Many households keep compost, cities collect compost, and we feel less guilty using disposable cutlery marked “compostable.” Throwing food waste into a pile of leaves and earth, letting it rot, and stirring it around has never been so fashionable.

However, not all composts are made equal. Some composts have the ability to suppress plant diseases such as phytophthora root rot. Plant disease suppression involves a three-way interaction between the microbes in the compost, the plant, and the plant pathogen. The ingredients from the compost can favor different species of microbes. The microbes transform the compost and produce various molecules and compounds (we can thanks microbes for the stench!). Lastly, the different molecules produced by the microbes (or the microbes themselves) can affect the plant and the plant pathogen.

Researchers from the Centro de Edafologia y Biologia Aplicada del Segura and the University of Alicante studied how compost composition affects its ability to control phytophthora root rot. They made four different compost recipes and monitored how the pepper plant fared against phytophthora root rot:

  • Compost A: pepper sludge (12.5%), pepper waste (12.5%), vineyard pruning waste (75%)
  • Compost B: pepper waste (16%), artichoke wastes (16%), vineyard pruning waste (68%)
  • Compost C: pepper sludge (19%), pepper waste (2%), garlic waste (2%), carrot waste (35%), almond shells (4%), vineyard pruning waste (38%)
  • Compost D: artichoke sludge (15%), artichoke waste (26.4%), vineyard pruning waste (50%), compost (8.6%)

Compost A and Compost B were the best suppressors of phytophthora root rot. The researchers compared the microbes and molecules the four composts. Of course, things are never simple when it involves hundreds of different species and molecules. It did not appear that a specific microbe contributed to the suppression of phytophthora root rot. The researchers did notice that the best suppressors of phytophthora root rot had many smaller molecules in the compost. The smaller molecules are an indicator that the microbes are active, breaking down larger components in the compost to smaller pieces. Thus, it seems that microbial activity in the compost is required for disease prevention.

Do we have a formula for the perfect compost? Not quite yet. The study of the microbial and chemical composition of compost is still in its infancy and this study was the first to characterize the metabolome of compost. The possibilities for compost studies are endless. We can vary the things we put into compost and different plants may benefit from different types of composts. Perhaps one day, we will have compost formulations designed to prevent and treat a plethora of crop diseases.

CC BY-NC-ND 4.0 Think again before you compost that by Jennifer Tsang is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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