- by Todd C. Wehner
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
- and Gary W. Elmstrom
- Sunseeds Co.
- 7087 East Peltier Rd.
- Acampo, California 95220
Early watermelon varieties were mostly inbred lines produced
commercially by open pollination of bulk-increased or hand-pollinated
breeder seeds. In the 1970s, large-scale production of diploid
hybrid seed began. Diploid hybrids have now taken over most
of the commercial production in North America, Western Europe,
Hybrid seeds are produced in the seed parent by pollination
from staminate flowers in the male parent. Hybrid production
can be by hand pollination using inbred lines grown in adjacent
rows in the field, or by planting the two parental lines
in an insect-proof cage. Pollinations are marked for later
seed harvest using tags or bags after pollination. Each
fruit will have 200 to 800 seeds, and fewer than 4000 seeds
are needed per acre of commercial production.
A less expensive alternative to hand pollination is to
plant the two parental inbreds in an isolation block. Staminate
flowers are then removed daily from the plants in the seed
parent rows to avoid self- and sib-pollination. All pistillate
flowers in the seed parent row that are pollinated during
the days of staminate flower removal are tagged for hybrid
seed harvest. Another solution would be to incorporate a
recessive seedling marker such as non-lobed leaf or the
glabrous gene into the seed parent. Seedlings resulting
from self- or sib-pollinations would have the marker and
could be removed from the planted field or removed from
the transplant flats to get 100% hybrid seedlings. Conversion
of the seed parent to a near-isogenic male sterile line
offers the possibility of hybrid seed production without
the work associated with the above three methods. However,
genetic male sterility requires that male fertile plants
be rogued out of the seed parent rows in the hybrid production
Seeds can be sorted after the seed cleaning operation by
size, weight, or density to increase the proportion of hybrid
seed in the lot. Diploid open-pollinated seed yields should
be higher than 251 lb./acre (average for United States in
1976-1977). Very good seed yields would be 400 lbs./acre.
Triploid seed yields average about 20-40 lbs./acre (about
10% what diploids would produce).
Most commercial watermelon seed production is located in
arid or semi-arid areas of the world such as western China,
Chile, Mexico, Thailand, and the United States (California
and Colorado). Arid conditions favor the production of high
quality, disease-free seeds. With the outbreak of bacterial
fruit blotch of watermelon in the late 1980s, seed production
in areas of low humidity and no rainfall has become even
more desirable in order to produce disease-free seed.
Sanitation is important at all stages of production. Workers
should wash their hands with antibacterial soap or rinse
them with 70% isopropyl alcohol before handling plants or
fruit and between seed lots. All equipment should be cleaned
and all soil and plant material removed before use in production
areas. Clean and disinfect harvesting tools and equipment
with alcohol or 0.5% NaOCl or Ca(OCl)2 between seed lots.
Sanitation, harvest, and control procedures for production
of foundation and stock (parent) seed should be at least
as stringent as that for commercial seed.
The process of growing watermelon seed crops is similar
to that for growing market crops except that site selection
is more critical. Choose a field that has not had any cucurbits
(watermelon, cantaloupe, honeydew, cucumber, summer or winter
squash, pumpkin, or gourd) in it for at least 2, but preferably
4 years. A field that has a history of fusarium wilt or
anthracnose should be avoided. Fields for open-pollinated
watermelon seed production should be isolated by at least
1 mile from other watermelon fields to prevent contamination
by outcrossing. Isolation also prevents disease spread from
fields containing watermelon and cantaloupe crops of unknown
origin or planted with seeds that have not been tested for
seed-borne disease. The production site should be as far
as possible from fields where bacterial fruit blotch occurred
the previous year to reduce contamination from leftover
debris. Wild cucurbits, such as citron and volunteer watermelons,
must be removed from a 1-mile radius surrounding the production
field to eliminate outcrossing and disease contamination.
Selection of parental seed from elite or foundation seed
is the first critical element of seed production. Use seed
that was produced in dry climates and has been tested to
be free of the pathogens causing gummy stem blight, watermelon
fruit blotch, anthracnose, and squash mosaic. Direct-seeded
plantings reduce the risk of seedling contamination in greenhouses.
If transplants are used, they should be produced in a greenhouse
that does not contain other cucurbits. Irrigation of transplants
in the greenhouse preferably should be from an ebb and flow
or a float system. Overhead irrigation of seedlings in the
greenhouse should be avoided. Greenhouses for transplant
production should have good air circulation and low relative
Drip or furrow irrigation should be used in the production
field instead of overhead irrigation to reduce leaf wetting
and disease spread. Roguing of off-type and diseased plants
within the field should be done throughout the growing season.
There are four useful stages for roguing. The first is before
flowering when vegetative characters are checked. The second
stage is at early flowering when morphology of undeveloped
fruit is checked. The third stage is when the developing
fruit are checked for trueness to type, and the final roguing
is confirming the external morphological characters of the
fruit to be harvested. Roguing for off-types is not effective
after pollination in a field for open-pollinated seed production.
It is only effective when fruit have been self or cross-pollinated
and the male has no off-types. Inspectors should be trained
to recognize variations in watermelon fruit blotch symptoms.
Preventative applications of copper fungicide can also
help in reducing fruit blotch contamination of seed. The
first spray should be 2 weeks before flowering. Application
of registered fungicides will reduce gummy stem blight seed
contamination. Seed should not be harvested from fields
where there is confirmation of fruit blotch or until the
possibility of fruit blotch is eliminated. Seeds harvested
from fields in which fruit blotch is confirmed or which
were adjacent to contaminated fields should not be used.
All fruit should be inspected by trained technicians for
symptoms that are suspected to be fruit blotch. All fruit
suspected of having fruit blotch must be discarded. No fruit
should be harvested from vines that have anthracnose or
gummy stem blight symptoms. When seeds of open-pollinated
fruit, and in some cases, hybrid fruit, are mature the fruit
are windrowed by machine. Windrowed fruit are picked up
by self-propelled vine seed harvesters that crush the fruit
and separate the seeds and pulp from the rind. For some
hybrid seed production, fruit are harvested by hand and
various sized seed extractors are used (Fig. 3.13). In either
case, the diploid seed slurry is transferred to bins where
it is allowed to ferment for 24 to 48 hours. During this
time the sugars and gelatinous material surrounding the
seeds are degraded.
Fermentation plus acid washing (1% hydrochloric acid) can
reduce the chance of seed transmission of fruit blotch.
Fermentation and acid treatment of triploid seed reduces
seed viability, so is not recommended. Seeds extracted from
tetraploid fruit for triploid seed production should be
washed immediately. Seeds are separated from pulp and juice
by washing in a rotary washer or flume system (Fig. 3.14).
Some seed lots are dried by heat from the sun. However,
higher quality seeds are produced using forced air warmed
by propane heaters. Seeds are placed on flat drying beds
or in large rotary dryers. Dry seeds are run through a mill
containing sizing screens that separates large seeds from
trash and small seeds.
All seed lots should be assayed for the presence of the
fruit blotch bacterium, squash mosaic virus, and gummy stem
blight pathogen by the best methods available. In Asia,
cucumber green mottle virus is a problem and is seed transmitted.
For fruit blotch, seedling grow-outs of at least 10,000
seeds per lot are currently used, but polymerase chain reaction
(PCR) techniques may provide more efficient and sensitive
methods. Coupling seedling grow-outs with PCR may be necessary
for some situations. Squash mosaic virus can be screened
with grow-outs. For gummy stem blight, seedling grow-outs
or blotter tests using a minimum of 1,000 seeds per lot
are recommended. However, PCR techniques may provide better
methods in the future. Commercial seeds should be treated
with a registered protectant such as Captan and Thiram before
sealing them into cans, bags, or packets. Seeds should be
stored in hermetically sealed containers at 6.5% moisture
content, and no greater than 10% moisture. Under favorable
storage conditions, seeds should last 4 years. To be salable,
germination of the seed lot must be at least 70%.