The issue of compatibility/seediness between different mandarin cultivars is a major concern for growers in California.

If a grower plants two compatible mandarin cultivars next to each other or nearby, there will be a lot of seeds in the fruit. Mandarin fruit with large numbers of seeds will be downgraded and receive lower returns for the grower.

To find out more about this issue, I conducted a pollination study in 2002 and 2003 that was funded by the California Citrus Research Board. I did a hand cross pollination study among different mandarin cultivars at the University of California Lindcove Research and Extension Center near Exeter, Calif.

The study included three types of mandarins:

Clementine mandarins such as Fina Sodea and Nules. Nules is the leading cultivar with large acreages of new plantings in California in recent years.

Afourer mandarin, also known as W. Murcott mandarin, is sold under the trade name Delite mandarin. Afourer mandarin is also one of the leading cultivars that have been planted in California in recent years.

The third type tested was Tahoe Gold mandarin and Gold Nugget mandarin, are both new seedless mandarin cultivars recently released by the citrus breeding program at UC Riverside by M. Roose.

Tahoe Gold mandarin, also known as TDE3, is a triploid mandarin and completely seedless even in a mixed planting. Gold Nugget mandarin is adiploid mandarin and also completely seedless in mixed plantings.

Clementine and Afourer mandarins are nearly completely seedless if they are grown in isolation. These two types of mandarin are known to have seeds if they are planted nearby, but the severity of the seediness is unknown.

The Clementine mandarins used in the study belong to a Clementine mandarin cultivar trial established by the late David Gumpf, Department of Plant Pathology, UC Riverside and Citrus Clonal Protection Program (CCPP). The Afourer mandarin belongs to a mandarin cultivar rootstock trial established by Louise Ferguson, Pomology Department, UC Davis. The Tahoe Gold mandarin and Gold Nugget mandarin are located in the CCPP foundation block.

Early conclusions

Based on the results from 2002 and 2003, some preliminary conclusions can be drawn and some results can be used as guidelines for future planning of new mandarins in California.

First, crosses between Clementine mandarins and Afourer mandarin were found to produce very high fruit set and a lot of seeds in the fruit. There were 30-40 percent fruit sets in 2002 crosses with an average of 23-25 seeds per fruit. The fruit set percentage was lower in 2003, but the cross between Nules Clementine and Afourer mandarin had an average of 32 seeds per fruit. The reciprocal crosses between Afourer and Clementine mandarins had lower fruit set percent, 28 percent in 2002,and an average 9-12 seeds per fruit.

The Fina Sodea Clementine by Afourer mandarin cross had 21 percent fruit set with an average 11 seeds per fruit in 2003.

The Clementine and Afourer mandarin are highly competitive and could cause large numbers of seeds in each other. All efforts should be made to keep these two types of mandarins from each other.

Large distances should be maintained between these two types of mandarins at all times to avoid seed production.

When the Gold Nugget mandarin was used as the pollen parent in crosses with Fina Sodea Clementine and Afourer mandarin, fruit set was zero in all crosses. Gold Nugget mandarin probably will not cause any seed in Clementine mandarin and Afourer mandarin, and it could be planted near them without concern for seed production.

The Nules Clementine × Tahoe Gold mandarin crosses gave 14 percent fruit set with an average 1.5 seeds per fruit in 2002 and 17 percent fruit set with an average 10 seeds per fruit in 2003.

However, by some unknown mechanism, Tahoe Gold is able to cause seeds in Nules Clementine. It is however, not compatible with Fina Sodea Clementine and Afourer mandarin in crosses.

Avoid planting nearby

Based on the preliminary findings, the results suggest that growers should avoid planting Tahoe Gold mandarin near Nules Clementine.

Tahoe Gold may cause some seeds in Nules Clementine mandarin.

I am growing seeds from the Nules Clementine × Tahoe Gold mandarin to determine what might be the mechanism for the seed production.

I will repeat some crosses in spring of 2004 at LREC and include other citrus such as lemon, grapefruit, Shasta Gold' mandarin (TDE2) and Yosemite Gold mandarin (TDE4) in the crosses.

(C. Thomas. Chao is an assistant Extension horticulturist in the Department of Botany and Plant Sciences at UCR).