Rootstocks Offer Production Attributes

Tomato Rootstocks Grafting

By Jessica Theisman, Associate Editor

California Ag Today recently spoke with Brenna Aegerter, a UC Cooperative Extension farm advisor in San Joaquin County, about grafting rootstocks to tomato plants.

“Rootstocks are cut below the cotyledons, while the scions are cut above the cotyledons at an angle,” she said.

These two plants are then clipped together and go into a high humidity healing chamber for about one week.

Aegerter explained that, “In the beginning, there is no light. They gradually increase the light because they do not want to stress the plants, and so if those two angles match … then everything grows back together.”

Each tomato plant can differ depending on the rootstock that it has been grafted to. Different rootstocks have varying levels of resistance to disease and pests. Rootstock resistance does have an effect on crop yield. If all of the fruiting varieties had the nematode resistance gene, they could potentially improve their yields.

There is great success rate on grafting these tomato plants.

“We have had pretty good success, 90 percent and up,” Aegerter said.

The remaining 10 percent is because the angles were not matched up quite right and there is not enough contact between the two tissues.

“Rootstocks are for the most part, hybrids between our cultivated tomato and wild tomato species,” Aegerter explained.

Wild tomatoes are used often to bring in new genetic material due to their diversity and natural resistance. Depending on the type of tomato that is being used, the resistance can differ.

The fruiting varieties are resistant, but are resistant to a shorter list of diseases.

“These root systems are are bigger, the crown is bigger, and sometimes they aren’t even purely resistant to the disease, but just by virtue of the fact that they grow faster, they can outgrow it,” Aegerter said.

Grafting Rootstocks on Tomatoes, A Growing Trend

Grafting Can Yield Bigger, Better Tomatoes

By Jessica Theisman, Associate Editor

Brenna Aegerter is a San Joaquin County Cooperative Extension Farm Advisor. She specializes in vegetable crops, and she’s working on tomato plant grafting for market tomatoes. “Right, they would be doing it at the greenhouse. They’d start off the plants like normal, except they’re planting both the rootstock and the regular fruiting variety, so you’re planting two seeds. Then, these little seedlings are cut in half and put back together with the root stock on the bottom” Aegerter said.

The tomatoes are then placed in a dark, humid chamber, which allows them to heal for about one week. Following this step, the plants are then moved back into the greenhouse. This process speeds up the growth cycle on these plants by about two weeks. When the tomatoes are ready to ship, they look the same as any tomato plant except; these have a plastic clip that helps support the graft union. “It’s really soft, like a biodegradable silicon,” Aegerter said.

There are a few reasons why this process is necessary. “We want to increase the vigor of the plants, and we want more fruit, more vine, and bigger fruit. This is for fresh market tomatoes,” Aegerter explained.

It is not necessary on processing tomatoes, as the grower wants total tonnage. “We’re looking for some resistance to some of these diseases that we have. The idea being that maybe we can reduce,” Adgerter said.

One way to avoid the soil borne diseases is to plant tomatoes with the grafting union above the soil. These tomatoes however, have a tall transplant and most of the stem is buried. There’s no way in current production practices to plant with the union above ground. “But even with the deep roots, the rootstock is vigorous,” Aegerter said.

This rootstock must be bred. Sometimes they are a wild species, but for the most part, they are hybrids of wild and cultivated tomatoes. They contain some characteristics of a wild tomato but have a tomato rootstock breeder. The breeder, Monsanto/Seminis, is working worldwide for different markets. “[Their] job is breeding rootstocks,” Aegerter said.

This practice is an emerging standard that is becoming more common in other parts of the country. This takes place in the Southeast such as North Carolina. They are growing stake tomatoes, which is a very different production system. They pick multiple times while growers in California pick once.

The greenhouses aren’t set up with automated grafting. All of the grafting is done by hand. “To bring that cost down to a price that we’d like to see, they’re going to have to be more automated. But how can they justify the decision to invest in very expensive automated grafting equipment, if they don’t know if the market is there?” she said.

Replanting Trees as Harvest Ends

David Doll on Replanting Trees

By Laurie Greene, Editor

As harvest comes to a close for many tree crops, the time for replanting trees is swiftly approaching. David Doll, a UC Cooperative Extension farm advisor in Merced County, said that if California receives significant rain this year, the replanting process in orchards would be more difficult.

“If we are potentially coming into a wet winter, it’s going to provide challenges in establishing new orchards,” Doll said. “In the case of heavy rainfall, it’s important to keep a few things in mind and plan accordingly. First, if we’re doing any type of soil modification, we need to get a little bit of moisture to help the soil settle.”

Doll said second step is ‘pulling’ berms—the small hills or walls of dirt or sand in an orchard created to divert rain and irrigation water from the tree trunk. He explained, “We want to pull them before the soil gets too wet. We don’t want to walk into a heavy soil field, such as clay or clay loam, and pull berms because in doing so do, we would actually slick that soil over and have to deal with compaction and future issues with the orchard.”

“Third, when we start planting our trees,” Doll said, “it’s important to make sure that we dig a proper hole with wet soils.” Doll warned if you don’t spend the time to dig a hole,  you can ‘glaze’ the soil or form a crust on the sides of the holes, particularly in clay soils, leaving a hard, compact surface that is impenetrable to young roots. He advised to fracture or scratch glazed soil on the sides of the hole with a shovel or rake before filling in to ensure proper root growth.

Doll also said that when planting, the graft union—the point on a plant where the graft is joined to the rootstock—needs to be kept aboveground. “Countless times I’ve seen people plant the graft union below the ground,” said Doll. “Or they’ll plant the tree, pull up a berm, and actually put the graft union below the ground. Keeping the graft union about one hand’s width above the soil line will ensure the graft union remains aboveground as the tree settles.”

“Lastly, if machine planting in very wet clay loam soil, clods [lumps] and air pockets may form,” Doll said. “That’s problematic. The same thing also may occur with hand planting. It’s important to make sure the planters are digging a properly-sized hole and the roots need to be sufficiently covered. The soil needs to be broken down and then replaced around the tree. Finally, to ‘tank’ the tree, apply about 4-5 gallons of water after replacing the dirt to reduce the air pockets and allow the tree to get a good, solid start.”