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Plants can get sick, but since they don’t walk around sneezing on each other, the things that infect them need some very weird strategies to spread.

Hosted by: Michael Aranda

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Crown gall
Christie & Gordon, 2015:
Mummy Berry
Cedar-apple rust

When you're sick, you might sneeze or cough or get a runny nose, all things that help the disease spread.  Plants get diseases too, caused by all types of infectious agents, bacteria, viruses, fungi, parasites, but sick plants don't get runny noses.  Since plants don't walk around sharing doorknobs, the things that infect them need very different strategies for reproducing and spreading, and some of these plant diseases are, well, bananas.

Crown gall is a cancer that affects over 600 plant species, from asparagus to apricot trees.  It's caused by a type of bacteria, but really the whole infection process is orchestrated by a plasmid that the bacteria carries.  A plasmid is a circular piece of DNA that exists independently of the bacterium's main chromosome.  Plasmids usually aren't essential to a bacterium's lifestyle and in fact, these bacteria can survive without it, but it does confer some benefits, like setting up the bacteria with food and a place to live, but really, it's all in service to the plasmids continued survival.

First, it makes the bacteria sniff out and move toward a vulnerable plant that has a wound near where the stem comes out of the ground.  Since fruit trees are often grafted on to different root stalk, they're especially vulnerable.  Once inside the plant, the plasmid inserts a piece of itself into the plant's DNA.  The genes from the plasmid change the way the infected plant cells grow.  The cells form a mass called a gall and they make opines, a type of food for the bacteria that the plant doesn't normally make, and get this, it is genes on the plasmid that enable the bacteria to take in and use these nutrients from the plant.

If plasmid-free bacteria come into the neighborhood, the plasmid can get its bacterial host to pass it on to them, turning the new newcomers infectious.  These bacteria can spread through the soil to new vulnerable plants.  Basically, it's not the bacteria's fault exactly.  None of this would happen without this ring of DNA, and that actually leads to a really cool and unique biological hack to help stop the spread of crown gall.

These days when plant nurseries graft young fruit trees onto root stalk, they add a different strain of the same bacteria to the site.  These bacteria carry a different plasmid and they basically out-compete the disease-causing type so it can't get a foothold. 

Mummy berry is a blueberry infecting fungus with a totally different set of strategies.  It hijacks both the plant and its pollinators to reproduce and it even has a strategy for surviving the winter when blueberry plants go dormant. 

The fungus' life cycle starts in the spring when the blueberry plant puts out new stems and leaves.  If a spore lands on a vulnerable young branch tip, it grows to cover it with a spore-producing film.  This film reflects ultraviolet light, releases a fruity odor, and makes sweet sticky liquid.  Basically, the fungus transforms the young stem into a fake flower.  It doesn't look like a flower to us, but it's more than enough to fool pollinators into visiting the chute and picking up spores inside of pollen, spores the pollinators then pass on to real blueberry flowers.  From there, the fungus can take root in developing fruit where they have access to tons of nutrients.  The infected fruits are doomed to never ripen.  Instead, they turn pinkish and wrinkled and fall to the ground.  They look sort of mummified, hence the name 'mummy berry'.  

The mummies spend the winter on the ground until spring, when they come to life and grow weird, alien-like, fruiting bodies which produce spores.  The resulting spores travel on the wind to infect new, young plant shoots.  The best way to stop these fake flower impostors is to target the mummies, so blueberry growers pick them up or bury them to stop this freaky disease from spreading, but it's not the only fungus plants have to worry about.

Cedar apple rust is a fungus that needs both cedar and apple trees to complete its life cycle.  Cedar and apple trees are so distantly related and their infection symptoms are so different that you wouldn't think it's all one disease.  It seems needlessly complicated, but there's a reason for it.  Apple trees get infected in the spring, whenever a fungal spore lands on a leaf, it forms a distinctly orange spot.  Eventually, fungal tubes grow on the underside of the leaves.  In the late summer or fall, the tubes release orange powdery spores that can only infect cedar trees.  

Since apple trees lose their leaves in the winter, the fungus can survive only if it spreads to an evergreen seeder.  Wherever a spore takes hold, the cedar tree grows a hard, round gall.  The gall protects and nurtures the fungus as  it slowly grows over the next 18 months, not through one, but two winters.  During the second spring, when the weather is right, the fungus puts on a show that's like Christmas ornament meets octopus.  Bright orange gelatinous tentacles burst out of the gall, sending out tons of powdery spores that can only infect apple trees.  The spores travel on the wind, moving back and forth between apple and cedar trees, and spreading to new hosts and the trees can be more than a kilometer apart.

Since spores from cedar trees can only infect apple trees and vice versa, the best way to control this disease is to keep these trees away from one another.  It also helps to clip away those cedar galls before they start to grow weird tentacles.

If plants could think, maybe they would think a runny nose was a super weird and freaky way to get sick.  They can't, as far as we know, but their symptoms sure seem bonkers to us.  Just like a runny nose, though, these diseases don't tend to kill their hosts, just inconvenience them in really, really odd ways.

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