Shinshu University discovers multiple plant-invasion strategies in anthracnose fungi

A research group including Associate Professor Hiroki Irieda of the Institute of Agriculture at Shinshu University, announced on July 14 that they revealed the diversity of invasion methods used by anthracnose fungi (Colletotrichum species), which cause severe damage to agricultural crops. While the formation of melanized appressoria was previously thought to be essential for anthracnose fungal infection, it was found that groups exist that can directly invade and infect before melanisation of appressoria or even prior to appressorium formation. This is expected to lead to the development of new control methods, such as novel agrochemicals. The results were published in Nature Communications on June 27.

Disease is said to cause 10% to 15% of global agricultural production damage. Filamentous fungi, a type of mold, accounts for most of it, making its control a globally important issue. Among them, anthracnose fungi and rice blast fungi have been studied for a long time. They spread spores via wind and rain to infect plants. Microscopic observations have revealed that spores attached to plant surfaces take a common strategy of forming a dome-shaped appressorium upon germination and invading and infecting through a needle-like penetration organ (penetration peg) from there.

Both feature the common characteristic that the appressorium becomes dark through melanisation, which provides turgor pressure elevation and robustness. Since the first report in 1982, melanisation had been considered essential, and there were no reported cases of infection by other methods. For rice blast, agents that inhibit melanisation are utilized in agricultural fields as effective agrochemicals.

Fungicides are used for anthracnose fungi while inhibitors are not, but it had been reported that inhibiting melanisation could prevent infection. Furthermore, anthracnose fungi exhibit host specificity. For example, Brassicaceae anthracnose fungi infect only Brassicaceae plants.

In this study, while investigating anthracnose fungi, the research group discovered a cosmos anthracnose fungus capable of infecting plants even when melanisation is inhibited. It was found that when a melanin biosynthesis inhibitor (carpropamid: CAR) that blocks melanisation was added to this anthracnose fungus, the appressorium became transparent and melanin biosynthesis was inhibited, yet it extended a penetration peg from the transparent appressorium to establish infection. After examining appressorial turgor pressure and plant invasion capabilities across various anthracnose fungi, they found that dependency on melanisation differed.

Next, they investigated whether non-cosmos anthracnose fungi exist that can invade plants even when melanisation is inhibited, by verifying the effect of CAR on several dozen types of anthracnose fungi held in their laboratory. As a result, it was revealed that the group of the acutatum species complex, including cosmos anthracnose fungi, established invasion and infection even when melanisation was inhibited. Although melanisation was inhibited by CAR in all appressoria, infection was established from transparent appressoria. It is thought that this ability was acquired during the evolutionary process.

It became clear that multiple anthracnose fungi exist that directly invade plants from the tips of hyphae extended without forming appressoria after spore germination. An invasion method not depending on appressoria was reported in only one case over 15 years ago, but it had been considered an exceptional case. At that time, it was reported to occur in an environment where sugars were present nearby. In this study as well, adding sugars led to invasion without forming appressoria. One strain was also discovered that exhibited a reaction similar to sugars when treated with pyroquilon, another melanin biosynthesis inhibitor. It also became clear that not only the acutatum species complex but also the gloeosporioides complex group tended to exhibit this invasion method.

Overturning the established theory, the possibility was demonstrated that a group of anthracnose fungi exists that selects invasion methods according to the surrounding environment.

Irieda stated: "Because melanin inhibitors are not used as agrochemicals against anthracnose fungi, it is unlikely that they emerged as resistant strains. In the first place, the acutatum species complex and gloeosporioides species complex emerged about 10 million years ago, whereas the use of agrochemicals inhibiting melanisation began in the 1980s. Because invasion by non-melanized fungi is faster than by melanized ones, and even faster if appressoria are not formed, there may be an advantage in being able to infect earlier. I believe we have obtained results that will serve as a foundation for future agrochemical development."

Journal Information
Publication: Nature Communications
Title: Breaking dependence on melanisation imparts diversity to a dogmatic invasion strategy of phytopathogenic fungi
DOI: 10.1038/s41467-026-74937-6