Cooperative Extension Part 2

Friends, family, and colleagues,

This is the continuation of our outing in Dongtai. You can find part 1 on a previous post.

After we talked to the grower at Dongtai, we traveled to another operation with a “new disease” in their operation. As extension folks, we love disease, but to really get an extension pathologist excited, use the words “new disease”. The symptoms were described as a swelling at the base of the rootstock plant and wilting of the scion plant. This was a problem only on the grafted watermelon, though his nongrafted melons had wilting as well. While we drove to the field, Dr. Paret and I speculated on what we might find. Is it a bacterium? A virus? Herbicide damage?

Typical operation in Dongtai, Jiangsu Province. Melons are grown under protected structures.

Grafting vegetables

As I mentioned in an earlier post, nearly all the watermelon in China is grafted. To graft a watermelon, the seedling is cut and then reattached onto a closely related species (gourd or pumpkin usually). The top portion (the watermelon) is called the scion while the bottom portion is called the rootstock. This is very popular in the US for tree fruit production to dwarf the plants to make harvesting easier. There is a lot of utility in grafting one plant onto another. You may find more information here.

The purpose of grafting in this context is to maintain resistance to a pathogen Fusarium oxysporum. This is a soilborne pathogen that infects the roots and colonizes the vascular tissue. The fungus, coupled with the host response, wilts the plant. The disease is appropriately called Fusarium wilt. However, Fusarium wilt is very host specific. Each species can be broken down further by forma speciales, which is latin for form species. This explains what the host of the pathogen is. For example, the forma speciales that goes to watermelon (Fusarium oxysporum f.sp. niveum) cannot infect closely related species, such as pumpkin or squash. So, if you have a problem with Fusarium wilt in your watermelon field, one solution is to graft your watermelon onto a pumpkin, gourd, or squash.

A grafted melon with the graft union in blue.
There was also some gummy exudation (red), which is a classic symptom of my disease

Now here is where it gets (disease) complex

However, Fusarium oxysporum often has help in infecting plants. A small roundworm, called a nematode lives in the soil and feeds on the root of plants. These create wounds which are exploited by the Fusarium to cause infection. This is known in my field as a disease complex, where the disease is more severe in the presence of another pathogen. Unlike the Fusarium, nematodes have very broad host ranges. They are not picky eaters. They can attack the roots of squash, pumpkin, gourd, watermelon and many more. By itself, nematodes can cause a lot of damage to crop roots resulting in wilting and stunted plants.

Investigative plant pathology! Crop Scene Investigation

I think you know where this is going. We get to the field and the grower points to one of the symptomatic plants. The rootstock is fat, but the scion is wilted and dying. Dr. Paret pulls up the plant and the first thing I notice are galls on the root. We have found our first clue. The roots are sparse and covered in swollen galls, which are characteristic of nematodes. When the nematodes feed, the roots respond by dividing their cells rapidly and expanding the cells they already have (known as hyperplasia and hypertrophy respectively). 

Courtesy: http://www.expert-nutrition.com/muscle-hypertrophy.html

Looking for more clues

The next step to a diagnosis is to cut the stem open with a knife to check the vascular tissue. We find that the grafted plants are clean. There is no visible discoloration which we would see if there was Fusarium wilt. The swelling was caused by the few roots left trying to push the water up, but not being able to make it past the graft union. The scion wilted while the rootstock swelled. How cool is that?!

Galling caused by root knot nematode

Stem swelling. Note the tiny amount of roots.
Notice that there is no discoloration in the vascular tissue.
This plant is not infected with Fusarium wilt.

The second “crime scene”

Next, the grower takes us to some non-grafted plants. He thought that maybe the rootstocks were defective, but the same thing was happening. We pull one up, with only a few roots (lots of nematode damage). We cut open the stem to be met by a brilliant orange discoloration! You have Fusarium wilt! Ok, we weren’t THAT excited, but it was close.

Classical symptoms of Fusarium wilt. Note the orange discoloration along the vascular tissue.

Delivering the news

We explain (though a translator, I don’t know the word for nematode in Chinese) about the problems this grower has and some possible solutions. He peppers us with many questions about rotating fields, what this means for his production, et cetera. The cool part about cooperative extension is because the grower is a captive audience, he is truly listening to what we have to say. What’s even cooler is his 17-year-old son is listening even more intently (and taking LOTS of photos of the symptoms), probably because he will someday inherit this farm and wants to know what problems he will face.

It is experiences like this one that make it so rewarding to work in the applied sciences. I think all sciences have merit and every finding benefits mankind in some way, shape, or form. The extension work we do isn’t simply cataloged in a journal that only a few academics will read. It’s disseminated into real world situations. So while we didn’t find any new disease, we did help a grower solve some problems and educated him (and his son) on possible solutions. That’s what I love about my job!

Dr. Paret explaining the symptoms which is then translated via Dr. Ren (grey shirt).

I also think it's amazing that you can travel thousands of miles to find the exact same problems that you see in your own home. Experiences like this one are extremely rewarding. Who knew two guys from Florida would find the same pathogens a world away?

Mason