The history of pawpaw improvement is primarily a history of dedicated plant hunters. Superior plants have been located in the wild and propagated. The importance of this activity should not be underestimated. There are many valuable varieties still left to be discovered and brought into cultivation. In addition to the contributions made by plant hunters are those made by plant hybridizers, most notably Dr. G.A. Zimmerman of Harrisburg, PA. Plant hybridizers not only select superior plants, but increase the probability of “finding” superior plants by choosing the parents for each group of seedlings to be selected.
The first concerted effort at pawpaw improvement by a public institution was begun in Maryland in the early 1980s by Neal Peterson and his collaborators. Since that time they have put together a germplasm collection consisting of many hundreds of accessions. They have also made many crosses among pawpaw selections and between several Asimina species. Plants resulting from these crosses are reaching fruit-bearing age and selection of superior plants will soon begin.
As indicated above, pawpaw improvement can be categorized into three (non-exclusive) groups:
1. Selection of existing plants with unknown parents – Plant hunting.
2. Selection of plants when at least one parent is known. This includes:
a) making crosses (both parents known) and planting the resulting seed for evaluation, and b) harvesting fruit from a desirable tree (male parent unknown) and planting resulting seed for evaluation.
3. Intensive program of improvement utilizing genetic and statistical principles for the choice of parents, traits to select for, and the design and conduct of evaluation trials.
Whatever the method of improvement, those making the selections must have a set of criteria in order to make their choices. Table 4 lists plant, fruit, and other traits to be considered (for details see Peterson, 1989; Thomson, 1974; and Ourecky and Slate, 1975). The actual importance placed on each trait will be determined by the selector.