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Excerpts from Jim Conrad's
Naturalist Newsletter

Entry dated January 14, 2024, issued from near Tequisquiapan; elevation about 1,900m, (6200 ft), ~N20.57°, ~W99.89°; Querétaro state, MÉXICO
MINT RUST

During the current drought I water the Peppermint, Mentha x piperita, each day, and now I see that I should have made more effort to avoid wetting the leaves -- pour the water directly onto the ground. For, above you see that the sprouts, seemingly overnight, have become heavily infected with the fungal disease known as Mint Rust, PUCCINIA MENTHAE.

The disease currently manifests mainly as pimple-like pustules on the leaves' undersurfaces. In areas with cold winters, when autumn cools to a certain point, infected mint leaves turn deep-chocolate brown. On my plants, so far just one leaf is showing this, seen below:

So far this year here in the highlands of central Mexico we have experienced no frosts, and there may be none the whole season. I'm find no information on how the disease affects plants under our temperate, possibly no-frost conditions. Using literature available on the Internet to work out the life cycle can be confusing because different terms for the same thing may be used, plus the plant behaves differently under different seasonal conditions. The life cycle as described on the current Wikipedia page for the disease is hard to follow and incomplete.

However, various sources do describe Puccinia menthae as an autoecious macrocylclic rust. "Autoecious" indicates that the species completes its entire life cycle on a single species, in our case on Peppermint plants. The term "macrocylclic" when applied to rust fungi explains that during its life cycle five spore phases are expressed. Worked out from a variety of sources, here's my simplified general life cycle for the disease on Peppermint plants not growing here in central Mexico, but rather where freezes occur:

TELIOSPORE PHASE

The fungus overwinters as relatively large, thick-walled, black teliospores occupying fallen leaves, mint stubble, rhizomes and the soil's surface. Teliospores are "2n," meaning that they carry genetic information from two types of Puccinia menthae, analogous to male and female. Though they're considered to be spores, they're not spherical; under the microscope, teliospores are vaguely sperm-like, bearing tail-like projections.

BASIDIOSPORE PHASE

After a cold-induced dormancy of at least twelve or so days, teliospores "germinate," undergoing the type of cell division called meiosis, during which the 2n number of chromosomes is halved -- reduced to 1n. The result of this cell division produces a new kind of much smaller spore, the basidiospore. Once basidiospores are dispersed onto Peppermint tissue, threadlike hyphae emerge, penetrate the plant's tissue, grow into the cells, and parasitize them.

SPERMATIA PHASE

When warm temperatures of about 20°C are reached, proliferating Puccinia menthae hyphae inside the Peppermint's cells come together to create small, pustular structures, usually on leaf undersides, known as spermatogonia or spermogonia. A spermatogonium is like a tiny roundish bottle beneath the leaf's epidermis, with the bottle's mouth opening at the leaf's surface. Spermatogonia are filled with special hyphae at whose tips a new kind of spore develops, known as spermatia. When a spermatium released from its spermatogonium encounters a receptive hypha in another spermatogonium, sexual union takes place between the cells. This recombination of genes is known as plasmogamy, and at this point our 1n cells engender 2n ones. These 2n cells aren't spores, but rather what may be called conidia. Conidia provide a non-sexual, vegetative manner of fast reproduction. When mature, spermatogonia exude a liquid in which conidia are present in suspension. This conidia-containing liquid may be transferred by flies onto new Peppermint tissue, causing new infections when the conidia develop into hyphae which grow into the plant's tissue.

AECIDIOSPORE PHASE

Conidia infections lead to the formation of pustules called aecia (singular aecidium), sometimes known as "cluster cups". In the above photos of pustules rupturing and releasing brown, powdery stuff, we're seeing aecia releasing aeciospores, also known as aecidiospores.

UREDINIOSPORE PHASE

When aecidiospores infect the plant's tissue, resulting hyphae lead to the formation of yet another kind of pustule, known as the uredium. If weather remains warm, uredia will produce urediniospores (also called uredospores or urediospores). Urediniospores differ from other of our spore types in that instead of leading to a completely new kind of infection, they can create new uredia like the uredia from which they themselves have developed. As long as warm weather and adequate moisture holds out, new urediniospore generations can continue being created by new uredia producing yet more urediniospores, again and again.

However, urediniospores can't survive cold temperatures. At a certain point when the weather is cooling down, urediniospore infection, instead of producing new uredia as during warm weather, produce yet another kind of pustule, known as telia. And telia produce the overwintering teliospores with which we began this life cycle.

Not only that, but various "strains" of Puccinia menthae have been documented, and each strain has its own preference of host or hosts. For example, in the 2016 "Report on Plant Disease, Mint Rust," issued by the Univeristy of Illinois Extention, it was said that a Puccinia menthae isolate produced from Oregano could infect other Oregano plants, as well as Greek Oregano and Sweet Marjoram, but could not infect Spearmint; all these species are members of the Mint Family.

Not all rust species develop through the above five spore phases all taking place on the same species. For example, Cedar-Apple Rust spends 18-20 months on cedar trees, then other kinds of spores are produced during four to six months on apple trees. Cone Rust takes two to three years of pine cone infection before spores are produced. Also, the prime host is a pine tree, but the fungus's entire life cycle requires time spent on an oak, during the teliospore phase.

Why do rust species bother with all these different spore phases? One can see the advantage of have overwintering teliospores, and urediniospores which can be reproduced in several fast-developing and self-reproducing warm seasons, but why the rest? From what I can see, there's no clear answer. In the 2018 study by Cécile Lorrain and others entitled "Advances in understanding obligate biotrophy in rust fungi," a whole subdivision has the title "The different facets of rust life cycles and unresolved questions about their evolution." "Unresolved" appears to be the current status of our question.

Whatever the reason, my Peppermint plant's disease has been a pleasure to learn about. Knowing what I now know about Mint Rust, it appears that either I need to get used to having diseased Peppermint, or else just compost the whole plant and not put out any more Peppermint in this area.