A Lasting Impact
Gale McGranahan was the first woman to head the Walnut Improvement Program at the University of California, Davis. She pioneered new breeding methods and led researchers in developing new walnut cultivars and rootstocks. McGranahan strove to develop techniques to improve walnut breeding efficiency, to develop a healthy breeding population, and to maintain a diverse germplasm collection for future breeders.
McGranahan shares her concerns about the challenges the walnut industry faces, along with the successes she believes will help it thrive.
What challenges do U.S. walnut growers face?
There's not enough water, and it's sometimes very expensive; burdensome regulations on safety, pollution, and applications of chemicals; competition from other countries, like Chile; new pests and diseases; and good walnut ground being used as sites to build shopping malls and houses.
What new challenges do you anticipate in the future?
The tumultuous, unpredictable weather resulting from global warming; thousand cankers disease - no one knows what the impact will be; saturation of the market; lack of labor; and more restrictions on chemicals.
What are the short-term goals of the program?
The Walnut Improvement Program seeks to breed walnut trees with an earlier harvest date. There is a glut of walnuts at the end of the season, because too many high-quality, late-harvesting Chandler walnut trees have been planted. Equipment and processors can't handle the load. After nearly 30 years of backcross breeding for resistance to blackline disease, we are nearly ready to release a new, high-quality, resistant cultivar.
Selection can be challenging and somewhat mysterious. By elucidating the genomics of walnut, we and our collaborators hope to increase the efficiency of breeding and selection.
We are working with pathologists and nematologists to identify, propagate, field-test and release rootstocks with resistance to crown gall disease, nematodes, and Phytophthora root and crown rot, but we need to do more.
What else should the program be exploring?
We should also pursue walnut blight resistance. Early-harvesting walnut trees are also early to leaf out, especially during our rainy spring, when trees are susceptible and the bacteria that cause blight can flourish. We also need to breed for better storage capability.
What breeding methods do you use?
Which have been most and least successful?
Walnut was the first tree crop to be genetically engineered in 1989. In my lab and in cooperation with Abhaya Dandekar's lab, we developed the methodology to insert novel genes into walnut. The industry had no interest in marketing transgenic walnuts. Our biggest mistake may have been to devote so much time to it.
However, marketing a rootstock with a transgene for disease resistance may be successful. The cultivar on top is not transgenic, nor does the inserted gene express itself in the cultivar. My colleagues are now beginning to undertake the necessary paperwork and food safety tests to release a new transgenic rootstock. The growers and nursery people are very excited, because the disease, crown gall, is a serious problem. The male-sterile hybrid rootstock does not shed viable pollen, and as a rootstock produces no nuts, there is little chance of the gene escaping into the wild.
Mostly, we use modified recurrent mass selection to improve cultivars. Backcrossing provides resistance to blackline disease. In both cases, the parents are selected according to our goals.
The first step is to collect catkins from the male parent just as they begin to shed pollen. We place the catkins on a screen over butcher paper and leave it undisturbed for 24 to 48 hours, until the pollen sheds onto the paper. We sieve and store that pollen in small vials, placed in larger tubes containing magnesium chloride to maintain the correct humidity.
In the meantime, before the female flowers open, when they are just visible, we remove catkins from the surrounding area and enclose the female flowers in pollination bags. We take every precaution to avoid their meeting up with stray pollen. The bags have a window. When we can see that the flowers are at peak fertility, we inject a puff of pollen from the selected male parent using a needle and syringe. We label the bags and wait 10 to 14 days, until the flowers in the bags are past fertility. Then we remove the bags, flag the shoot with color-coded tape corresponding to the identity of the parents, and wait until just before harvest.
Nuts are collected before they abscise from the tree, then germinated and grown in a "seedling block." After several years, the seedling produces nuts and we begin the evaluation process. Growers join us in data collection and evaluation. The decision to release and patent a new cultivar is a joint decision between walnut growers, nursery growers, farm advisors and other UC faculty.
How will your work impact growers of the future?
We've released eight cultivars and two rootstocks so far. One of the cultivars is very early to harvest and therefore very popular. Another, recently released, has an early midseason harvest and will certainly be popular. Another has a red seed coat and was designed for niche markets. Two were selected for low blight scores; one of these is gaining in popularity, but the other harvests too late. We have also released two rootstocks with the help of many collaborators: one very vigorous and tolerant to nematodes, the other with resistance to a very damaging crown and root rot. The demand for these is very high.
I have retired, but the offspring of the last crosses I designed are still in the seedling stage, so my work should have an impact more than 10 to 20 years into the future.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.