Adventures in fruit breeding

 

Approximately 1000 hybrid grape seeds: (V. riparia L50-s x Trebbiano) by O.P. The material parent does not produce pollen so these are all out-crosses.

Jim Cummins, a retired breeder of rootstocks for fruit trees once made the statement "You really need to produce at least a thousand seedlings of any one cross to know what the potential is."

Breeders of fruits, both amateur and professional were taken aback by the statement.

The reasoning behind the statement is pretty clear.

Suppose you have ten characteristics that are critical to a successful introduction. In no particular order let me toss out:

1. Bears soon after planting
2. Tree bears enough fruit to be economically viable
3. Fruit has high sugar
4. Fruit has appropriate tartness
5. Fruit has great texture
6. Fruit has exceptional aromatics
7. Fruit is hard enough to ship
8. Fruit is brilliantly colored
9. Tree resists three major diseases of economic consequence
10. Tree is resistant to winter cold

Notice that I cheated with Number 9.

Now suppose you have two parents that you think represent a marriage-made-in-heaven in the sense that each parent is bringing five of those attributes to the cross.

If each trait is controlled by a single, dominant gene then the chances of one attribute being handed down to the seedling is precisely 50% and the chances of any two, specific attributes is 0.5 * 0.5 or 25%.

The chance of all ten attributes being passed on is 2^10 or one-chance-in-1024. That is for attributes that are controlled by a single, dominant gene (the simplest case).

If the attribute is controlled by a recessive gene then you will never be able to achieve that attribute in the offspring if the other parent doesn't have that recessive gene lurking in its gene-pool. Never.

And a great many attributes (taste, disease resistance and cold-hardiness, for instance) are controlled by many, many genes.

You will only have the very coarsest of approximation of the average of what a cross will produce if you only grow-out 10 or 20 seedlings.

The art-of-breeding

The art-of-breeding fruit trees and vines becomes a game of how cheaply can you produce hybrid seeds and then how quickly and how inexpensively can you kill off the progeny.

For example, it is possible to screen for a multitude of bacterial diseases before the first leaves appear. Soil known to be infected with a cocktail of Phytophthora genus bacteria is spread over the growing media where the seeds were very heavily sown. When the seeds are just starting to push their first, real leaves the media is flooded to just cover the tallest plant. In some cases the tiny seedlings are sand-blasted to ensure the bacteria has access to raw tissue. The water is drained after a week and the survivors are not only highly resistant to Phytophthora but also many other bacterial diseases that impact the bark and roots.

This level of screening would not be difficult for an amateur to do in a stock watering-tank and can result in 90%-to-99% of the progeny being discarded within three weeks of when they broke the soil.

The surviving seedlings are repotted and then run from chamber-of-horrors to chamber of horrors. The amateur equivalent is to pre-position the stock tank beneath a Typhoid Mary variety where inoculum will drizzle down on your baby seedlings. In my case, my least healthy grape variety for foliage diseases is Swenson Red.