Chapter 20: Phylum Basidiomycota: Order Uredinales—The Rust Fungi

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Welcome to the Deep Dive.

We sift through fascinating sources to bring you the really important, surprising stuff, the knowledge nuggets you need.

Today we're plunging into a world that's often unseen, but wow, incredibly powerful, ancient even.

We're talking about fungi,

but not just any fungi, a specific,

really impactful, and honestly kind of sneaky group.

Our mission, like always, is that shortcut to being genuinely well -informed, packed with those aha moments, hopefully with a bit of fun along the way.

So what's our focus?

It's the rust fungi.

These guys, part of the bustidium icota, order your red niles.

They're everywhere and their history is wild.

I mean, can you imagine the ancient Romans?

They were so hit by crop diseases from these rusts they thought, oh god, Robigus was behind it all.

They even had festivals, the robergalia, just trying to appease him.

That's how far back this goes.

And that impact, it's precisely why they're still so significant.

I mean, economically, they are some of the most destructive plant pathogens out there, huge damage to vital crops, forests worldwide.

But it's more than just the economic side, important as that is.

They're also with the fantastic model system for understanding really fundamental biology, things like development, genetics, those intricate host -parasite interactions.

There's just so much we learn from them that applies much more broadly in biology.

That's a really crucial point.

So yeah, over the next few minutes, we're going to unpack all that.

They're unique structures.

They're life cycles, which are, let's be honest, legendary for being complicated, how they actually infect plants so effectively and their bigger impact.

Food, forests, the whole picture.

Okay, so what exactly are these microscopic troublemakers, the ones that have been bugging farmers for literally millennia?

Well, first thing, they are all obligate parasites.

That means they have to have a living plant host, period.

They can't just, you know, live on dead leaves or something like lots of other fungi.

And here's a key difference right away.

Unlike many other Basidiomycetes, they don't make mushrooms.

Those visible Basidiocarps?

Nope, none of that.

Exactly.

No little toadstools.

Yeah.

And their economic toll, it really can't be overstated.

We're talking staggering losses globally.

I think black stem rust of cereals caused massive historical famines, basically.

White pine blister rust, devastating for forests.

And coffee rust, that would literally change agricultural history in places like Ceylon, now Sri Lanka, forced a whole switch from coffee to tea.

Then you've got asparagus rust, bean rust, cedar apple rust.

They're just incredibly pervasive.

Wherever the hosts grow, you'll likely find a rust fungus specific to it.

And structurally, they're quite distinct too, right?

They grow as this network of threads, mycelia, weaving through the plant.

Can you walk us through the main forms this mycelium takes?

What's happening in each phase?

Absolutely.

So there are two main growth phases.

First, you have the primary mycelium.

Here, each little compartment in the fungal thread has just one nucleus.

A single genetic blueprint, sort of.

This is the stage where there are specialized mating structures, the sex organs develop.

Then you get the secondary mycelium.

This is often the more aggressive phase.

Now, each compartment has two compatible nuclei working together.

And how they manage this is fascinating.

Instead of the clamp connections you see in many other related fungi,

rusts have this coordinated division process, ensures each new cell gets that pair of nuclei.

And it's this two nuclei, or dicariotic mycelium, that produces most of the spores that spread the disease.

Okay, so those are the threads inside.

How do they actually eat?

Get nutrients from the host?

Right, their feeding strategy is really specialized.

They grow between the host cells, intercellularly.

Then they produce these distinctive structures called hostoria.

Think of them like tiny specialized probes for absorbing nutrients.

A hostorium penetrates the host cell wall, but then it cleverly pushes in the host cell's plasma membrane without actually breaking it.

It invaginates it.

So it's extending into the cell's space, essentially, without rupturing that final living barrier.

Allows it to siphon off nutrients very efficiently, like a stealthy little straw.

Wow, and the symptoms they cause.

They sound really varied.

It's not just one look.

You might see strange swellings, right?

Gulls, like lumps on the plant, or sunken dead spots called cankers.

They can mess up branching too, creating these dense tangles, people call it witch's brooms.

And just general misery for the plant, stunting yellowing leaves.

That's chlorosis, right?

All signs of a plant under serious attack.

Yeah, a whole range of symptoms.

But what's truly astonishing, and something that just blew my mind when I first learned about it, is how some rusts literally reprogram their hosts.

Take Puchinia manoica.

It infects arabis plants related to mustards.

This fungus stops the host plant from flowering normally.

Instead, it makes the infected leaves look, feel, and even smell like the flowers of other, completely unrelated plants.

Shape, size, color, even nectar production.

They become fungal pseudoflowers.

Wait, pseudoflowers.

So the fungus makes fake flowers using the host's leaves.

Exactly.

It's incredible mimicry.

Why?

What's the point of these fake flowers?

Well, think about what flowers attract.

Insects.

So the insects visit these fake flowers.

Thinking they're getting nectar.

And instead, they pick up fungal spores.

The fungus basically hijacks the pollination system to spread itself.

So insects are tricked into being fungal mail carriers.

That is quite the biological scam.

It's not just mimicry, it's manipulation on a whole other level.

It really is.

It shows the amazing evolutionary lengths these parasites can go to.

It's manipulating the host, manipulating the environment, manipulating other organisms, all for its own survival and reproduction.

A remarkable example of parasitic control.

Okay, wow.

Let's try and unpack this.

The life cycles.

Rust fungi are just famous for being complicated and really flexible too.

Often involving multiple stages, sometimes multiple hosts.

It can feel a bit like biological chaos at first glance.

It can seem that way.

A key concept to grasp is heteroecism versus autoecism.

Heteroecious rusts need two different unrelated host plants to complete their life cycle.

It's like they need two homes.

The classic example is Puchinia graminis, black stem rust.

It cycles between barberry bushes, which are dicots, and grasses like wheat, which are monocots, very different plants.

Or Cranartium rubicola, white pine blister rust.

That one jumps between white pines, which are gymnosperms, conifers, and currants, or gooseberries, flowering plants, angiosperms.

They do autoecious rusts.

They do everything on just one type of host plant.

Simpler in a way.

Right.

So to make sense of this complex relay race, mycologists use a system.

Five distinct spore stages, labeled with Roman numerals, 0 to 5e.

Let's walk through them.

What does each stage do?

Okay.

Stage 0.

Spermagonia.

This sounds like the start of the sexual phase.

It is.

Think of it as the fungus sending out its mating signals.

These structures produce tiny male sex cells called spermatia and also special receptive hyphae, kind of like female structures.

And what's really neat is they often ooze this sweet, sticky, fragrant nectar.

Nectar from a fungus.

Why?

To attract insects.

Just like with those pseudo flowers, most rust fungi are meaning they're self -incompatible.

They need genetic material from a different mating type.

So insects visit, get sticky, and transfer spermatia between compatible spermagonia, kicking off the next phase.

Okay.

So insects play matchmaker.

What happens after that?

Stage I.

Stage I.

These usually form a sort of blister -like structures, often brightly colored, like orange or yellow cups.

Inside, they produce chains of ischiospores.

These spores are cariotic.

They have those pure nuclei now.

And very often in those hetero -ecious rusts, these ischiospores are the ones that infect the other host plant.

They bridge the gap between the two hosts.

Got it.

So stage urine often happens on one host, then stage II comes next, maybe on the other host.

Often, yes.

Stage II.

Urodinia.

They produce urodinia spores.

This is often called the repeating stage, or summer stage, because a single infection can produce multiple

crops of these urodinia spores in one growing season.

They form in these reddish -brown pustules that actually burst through the host's surface, the epidermis.

This makes infected plants look literally rusty, hence the name rust fungi.

And these spores are crucial for rapid spread within that host population.

Wind carries them, they infect more plants, make more urodinia spores, it explodes.

Okay, the rusty explosion stage.

Then what?

Stage III.

Stage III.

Telia.

These produce teliospores.

As the season winds down, the fungus often switches from making urodinia spores to teliospores.

These are typically thick -walled, often dark -colored, almost black.

They look different from the urodinia.

And critically, inside the teliospore is where karyogamy happens.

The two compatible nuclei finally fuse together, becoming truly deployed.

For many rusts, these tough teliospores are the overwintering stage.

They survive the cold or dry season.

The survival pod.

Okay.

And the final stage.

Stage IV.

Stage IV.

Basidia.

Producing basidiospores.

When conditions are right, that deployed teliospore germinates.

It doesn't form a mushroom, but a short structure called a promycelium.

Inside this promycelium, meiosis occurs.

That fused deployed nucleus divides back into four haploid nuclei.

Each of these develops into a basidiospore, which is usually thin -walled and often forcibly ejected, shot off into the air.

These basidiospores are the start of the cycle again, typically infecting the host where stage zero occurs.

And those basidiospores, they have options.

Right.

If they land on the right host, they can germinate directly and infect.

But if they land somewhere unsuitable, say on the ground or a non -host leaf, some can do what's called repetitive germination.

They basically produce a little stalk and form a new secondary spore, like a little escape pod, giving them another chance to get airborne and find the right host.

Pretty clever survival trick.

Wow.

Okay.

That's a lot of stages.

Let's try and tie it all together.

Can we walk through that classic example?

Puchinia graminis, the black stem rust of wheat.

See how all five stages play out.

Perfect example.

It really shows the whole heteroecious cycle beautifully.

Okay.

So where does it start?

Let's say end of the season on a wheat field.

Right.

You've got infected wheat stubble leftover after harvest.

On that stubble are the telia, stage three, containing the dark, thick walled teliospores.

These are diploid, remember, and they overwinter right there.

Okay.

Winter passes, spring arrives.

In spring, those teliospores germinate.

They form the promycelium, undergo meiosis, and produce four haploid vasidiospores, stage four.

Two will be one mating type, say plus, and two the other.

These tiny vasidiospores are shot off, carried by the wind.

And then you need to find, not wheat, right, the other host.

Exactly.

They need to find a barberry bush, the alternate host.

If they land on a susceptible barberry leaf, they germinate and infect it.

Inside the barberry leaf, they grow as primary haploid mycelium, and soon form spermagonia, stage zero, on the upper leaf surface.

These release the spermatia in that sweet nectar.

Then the insects come along.

Extracted by the nectar, yep.

They move between different spermagonia, transferring spermatia.

If a plus spermatia reaches a receptive hypha of a mating type, fertilization or plasmagmy occurs, digeritization happens.

So now we have the two -nuclei stage, starting on the barberry.

Correct.

Following that, on the lower surface of the barberry leaf, escha, stage one, develop, often visible as those little orange -yellow cluster cups.

These escha produce masses of dicariotic aciospores, chains of them.

These rupture out and are released into the wind.

And these spores are aiming for wheat.

Yes.

The wind carries the aciospores, potentially long distances, to the primary host wheat plants.

If an aciospore lands on a susceptible wheat plant, it germinates, infects, and grows as a dicariotic mycelium within the wheat tissues.

And this is where the rust really takes off on the wheat.

This is it.

That dicariotic mycelium produces uredinia, stage two, which bursts through the wheat's epidermis, releasing huge numbers of reddish -brown urediniospores.

This is that repeating stage.

Urediniospores infect more wheat, make more uredinia, more urediniospores, causing the epidemic spread, turning fields rusty during the growing season.

Okay.

And then as the wheat matures, the season changes.

Exactly.

The fungus switches gears.

The uredinia gradually convert to producing the dark, thick -walled teliospores within telia, stage three, on the maturing wheat stems and leaves.

And those teliospores are ready to overwinter, surviving on the stubble, completing the cycle, ready to start again next spring.

It's an incredible journey.

It really is.

Such intricate coordination between hosts and spore stages.

Now, how they actually get in.

Yeah.

Especially those urediniospores landing on wheat.

That infection process itself is fascinatingly precise.

Here's where it gets really interesting on a micro level.

Okay.

So a urediniospore lands on a wheat leaf.

What happens?

It can't just punch through anywhere, right?

No, it's much more guided than that.

It starts with what we call the recognition phase, the germ tube, the little thread that grows out from the spore.

It doesn't just grow randomly.

It actively senses the leaf surface.

It responds to physical cues, the pattern of wax crystals, the tiny ridges on the epidermal cells.

These guide it, almost like it's reading Braille.

Reading the leaf surface.

Yeah.

Looking for something specific.

Exactly.

It's usually looking for a stoma, one of those tiny porous leaves used for gas exchange.

It tends to grow perpendicularly across the rows of epidermal cells until it hits one.

And when it finds a stoma?

When the tip of the germ tube contacts the lip of a stermatal opening,

it stops growing forward and undergoes this amazing transformation.

It swells up, differentiates, and forms a specialized structure called an apresorium.

Think of it like a grappling hook or an anchor that forms right over the entrance.

And it's incredibly sensitive to the topography.

People have done these ingenious experiments using plastic replicas of leaf surfaces with tiny ridges mimicking the guard cells around stomata.

And the fungus would form apresoria only over ridges that were the exact height, like half a micrometer high.

It's responding to incredibly subtle physical signals.

Wow.

So it needs that precise signal to form the anchor.

What if it doesn't find a stoma?

If the germling wanders around too long, doesn't find a stoma, and doesn't form an apresorium within a few hours, it usually just exhausts its resources and dies.

It's a critical step.

Okay.

So it finds the stoma, forms the apresorium anchor.

What's next?

How does it get inside?

This leads to the signal phase.

A tiny narrow tube called a penetration peg grows down from underside of the apresorium right into the stomatal pore.

Once inside the air space beneath the stoma, this peg expands to form a sub -stomatal vesicle, a small sac.

From this vesicle, an infection hypha starts to grow out into the leaf tissue.

And during this phase, there's believed to be this crucial molecular crosstalk.

Signals exchange between the fungus and the host cells.

The fungus is likely actively suppressing the plant's initial defense responses, sort of

the feeding starts.

Then comes the parasitic phase.

The fungus is established and needs to get nutrients.

The infection hypha grows between the plant cells.

It forms a specialized cell called a hostorial mother cell, which attaches firmly to the wall of a host cell.

From this mother cell, a very fine penetration peg grows through the host cell wall.

And then remember, it invaginates the host's plasma membrane to form that hostorium inside the host cell.

This hostorium is surrounded by a modified host membrane, the extra -hostorial membrane, and there's often a neck band structure that seems to seal the connection.

We assume this is where nutrient uptake happens, likely amino acids and sugars flowing from host to fungus, but the exact mechanisms, the transporters involved.

That's still an active area of research.

It's a sophisticated feeding interface.

Such a detailed, almost surgical process.

It's incredible.

Now, all this complexity, the different mating types you mentioned, how does that play into genetics and how we even classify them or fight them?

Right.

The genetics are key.

As we touched on, most rusts are heterothallic.

They need those two different mating times, often just called plus and blank, for the sexual cycle to proceed.

You need a plus nucleus and a nucleus to eventually come together in that dekaryotic stage, initiated often by those insect transfers at the smorgonia.

These nuclei coexist, divide together in the dekaryotic mycelium, finally fuse karyogamy in the teliospore, and then meiosis in the promycelium segregates them out again into the bastidiospores.

It drives genetic recombination, creating new variations.

And this variation matters a lot when it comes to which plants they can infect, right?

That's where forme speciales come in, special forms.

Exactly.

Forme speciales, often abbreviated FSP, are crucial.

These are strains of a rust fungus that look morphologically identical.

You can't tell them apart under a microscope necessarily.

But biologically, they are specialized to infect only certain host species, or genera.

Like, Pochiniograminus FSP tritigae infects wheat, while Pochiniograminus FSP avany infects oats.

Same species, different special form, different host range.

It's a vital classification for understanding disease.

So, a special form targets a specific type of plant.

But even within that, there's more variation.

Races.

Precisely.

Within a single forme speciales, you can have different races.

Races are groups of fungal individuals, biotypes, that differ in their ability to infect specific cultivars or varieties of that host plant.

So one race of wheat stem rust might infect wheat variety A but not variety B, while another race infects B but not A, and a third might infect both.

This is hugely important for plant breeders trying to develop resistant crops.

And it led to H .H.

Flohr's foundational gene -for -gene concept back in the 1940s, studying flax rust.

The gene -for -gene idea?

What's the gist of that?

The basic idea is that for each gene conferring resistance in the host plant, an R gene, there's a corresponding gene in the pathogen controlling virulence, or avirulence, an AR gene.

So it's like a locking key.

If the plant has the right lock, R gene, and the pathogen has the corresponding key, A of right gene product, the plant recognizes the pathogen and triggers a defense response, usually.

And that defense response, what does it look like?

How does the plant fight back?

Often, this recognition leads to an incompatible interaction, resulting in hypersensitivity.

The plant cells right around the infection site rapidly die.

You might just see tiny deadspots, little flecks, on the leaf.

Since rust fungi are biotrophs, they need living cells to feed this localized cell.

Death effectively starves and stops the fungus in its tracks.

It's a very effective resistance mechanism.

As you said, usually, does it always work?

Ah, well that's the catch.

Because these fungi reproduce so quickly, especially in that uridinial stage, and because of sexual recombination, new races are constantly evolving.

A new race might emerge that has lost or altered its Avar gene, so the plant's R gene no longer recognizes it.

The key changes.

And the lock doesn't work anymore.

The fungus becomes virulent, the resistance breaks down.

It's a constant evolutionary arms race.

So how do we manage these constantly evolving threats?

What are the control strategies?

Well, historically, for heteroecious rusts like wheat stem rust, barberry eradication was a major strategy.

Removing the alternate host breaks the life cycle, preventing the sexual recombination that generates new races on the barberry.

This was implemented widely in Europe centuries ago, and in North America starting in the early 20th century.

It definitely helped reduce the frequency and severity of epidemics.

But it's not a silver bullet.

Uridinial spores can still overwinter in warmer regions and be blown northwards on the wind each year, bypassing the need for the barberry host locally.

So eradication helps, but doesn't solve it completely.

What else?

Breeding resistant plants.

Absolutely.

Breeding for resistance is the cornerstone of modern control.

But instead of relying solely on those single major R genes that new races can overcome quickly, so -called boom and bust cycles,

breeders increasingly focus on developing cultivars with more durable partial resistance,

sometimes called slow rusting.

These cultivars don't necessarily stop infection completely, but they significantly slow down the disease development.

Maybe fewer infections establish, the time between infection and new spore production, the latent period is longer, and fewer spores are produced per pustule.

The overall effect is much less disease buildup in the field, keeping losses below economic thresholds.

And importantly, this type of resistance seems harder for the fungus to overcome.

It's more durable over time.

That makes sense.

Fighting smarter, not just harder.

Now you mentioned classifying them can be tricky because of all these life stages.

Absolutely.

Taxonomy has been a challenge.

Because rusts are so polymorphic, having all these different spore forms, spermagonia, asia, uridinia, telia, basidia, and because you often only find one or two stages, especially the asexual uridinial stage,

it's been difficult to link them all together.

Traditionally, classification is based on the morphology of the teliospore, the sexual stage.

But if you only find, say, an eschel stage, you might not know which teliospore belongs to.

Historically, mycologists created artificial categories called form genera, like acidium, urato, chioma, just as temporary placeholders for these asexual stages until the full life cycle was discovered.

So like fungal John Doe's until they could figure out the whole story.

Is that getting easier now?

Much easier, thankfully.

Modern molecular techniques analyzing DNA sequences are revolutionizing rust taxonomy.

We can now compare DNA from different spore stages or from fungi and hosts and figure out their relationships much more reliably, confirming or correcting connections based on morphology alone.

It's helping to build a much more accurate and natural classification system.

That's great, because understanding these intricate details, it's not just academic, is it?

It has huge real world consequences.

Absolutely enormous.

We mentioned coffee rust, hemorrhoea, vastatrix.

Its arrival in Ceylon in the 1860s wiped out the coffee industry there within a couple of decades, forced a total shift to tea cultivation, and it's still a massive problem today in Latin America and other coffee regions.

It requires constant vigilance, fungicide sprays, developing resistant varieties.

It drastically impacts the livelihoods of millions of farmers.

Repeated infections weaken the trees, reduce yield, and can eventually kill them.

A single fungus changing beverage history and economies.

Wow.

And the pine rusts, also a big deal.

Huge deal in forestry.

White pine blister rust, cornarchium rubicula is another devastating example.

It's native to Asia, was accidentally introduced to Europe, and then to North America around 1900.

It infects five -neagle pines like eastern white pine and western white pine, sugar pine, crucial timber species.

It forms cankers on the stems that girdle and kill the trees, especially younger ones.

Massive efforts went into trying to eradicate its alternate hosts, currants and gooseberries, rive species, and into breeding -resistant pines.

It fundamentally changed forest ecosystems in many parts of North America.

Introduced pathogens can be so destructive.

Are there native rusts causing problems, too?

Definitely.

Fusiform gall rusta pine, caused by cornarchium corcum FSP fusiform, is native to the southeastern United States.

It cycles between oaks, alternate hosts, and southern pines like loblolly and slash pine.

It causes these large spindle -shaped gall swellings on the main stems and branches.

These weaken the wood, make trees susceptible to wind breakage, reduce timber value significantly.

And modern forestry practices, like planting vast areas with susceptible pines, have actually exacerbated the problem.

So our own practices can make things worse sometimes.

Any other interesting pine rust examples?

Well, there's western gall rust, endocrinarchium harknessii.

What's interesting is that while it's related to the others, it primarily spreads from pine to pine.

It seems to have sort of short -circuited the typical heteroecious cycle involving an alternate host, an example of how life cycles can simplify, too.

It's just amazing.

So as you can hear, digging into these biological details, it's not just for biology class.

It's absolutely vital for protecting our food supply, managing our forests sustainably.

And sometimes, like with those pseudo -flowers, it just uncovers these mind -blowing examples of evolution and interaction in nature.

So what does this all mean?

Our deep dive today?

Well, it shows rust fungi are incredibly diverse, incredibly impactful plant parasites.

Obligate parasites, remember.

They're life cycles, just marvels of biological complexity.

Multiple hosts, multiple spore stages, all intricately linked.

They use these unbelievably sophisticated ways to infect their hosts, literally reading the plant surface, disarming defenses, and the ongoing battle against them.

It really highlights both nature's incredible adaptability and our own human ingenuity in agriculture, forestry, and science.

Absolutely.

And perhaps a final thought to leave you with.

Even after centuries of studying them, these seemingly simple microscopic fungi continue to surprise us.

They reveal new layers of complexity, new evolutionary strategies.

Who knows what other secrets they still hold?

Secrets that might even revolutionize our understanding of life itself in unexpected ways.

A truly fascinating thought.

Thank you so much for joining us on this deep dive into the world of rust fungi.

We really hope you feel a bit more well -informed and maybe, just maybe, a lot more curious about the microscopic forces that are constantly shaping our planet.

Until next time, keep exploring, keep questioning, and keep diving deep into knowledge.