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July 23, 2018

Tomato Seed Production: Quick Reference

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Tomato Seed Production: Quick Reference - Organic Seed Alliance

Figure 1. The tomato flower on the left will self-pollinate. The flower on the right has an exerted style and may cross-pollinate.

Tomato Seed Production

Tomato plant spacing is affected by the growth habit of the variety. The three main growth habits are:

Plant spacing
Growth habit Description Between row In row
Indeterminate At least 3 leaves between 2 adjacent inflorescences on the same branch 48-72″ 24-36″
Determinate One or two leaves between 2 adjacent inflorescences on the same branch 36-72″ 14-24″, staked
Determinate compact Concentrated set for processing 36-72″ 14-24″, staked

Tomatoes like humid, warm to hot summers. Pollination and seed formation favor temperatures between 80-90°F. Tomatoes prefer sandy to clay loams that are well drained with high fertility for nitrogen, phosphorus, potassium, calcium, and magnesium. Soils need to have a high water-holding capacity, but overhead irrigation can cause disease spread and should be avoided when cultivating tomatoes. Tomatoes have perfect, self-fertile flowers. They are primarily self-pollinating, but can outcross depending on several factors. Solanum lycopersicum and Solanum pimpinellifolium (currant tomato) will cross-pollinate. Nearly all lycopersicum tomato varieties developed after 1950 are self- pollinating due to their flower structure (the stigma and style are enclosed by a fused anther cone). Some heirloom varieties produce flowers with exerted styles or protruding stigmas, which facilitate a higher occurrence of outcrossing and cross-pollination.

Figure 1. The tomato flower on the left will self-pollinate. The flower on the right has an exerted style and may cross-pollinate.

A tomato flower with an exerted style or protruding stigma may have an increased chance of cross- pollinating, but outcrossing will depend on other factors such as insect pollinator availability. In addition, long days, high temperatures, or low humidity can cause the style to elongate and favor cross- pollination. Besides flower structure, the types of bees present and their behavior on the blossoms, isolation distance from other tomatoes, the presence of barrier plants, and the planting pattern (rows vs. blocks) can affect the amount of outcrossing. The only way to know how much outcrossing is to grow different varieties together and regrow the seeds from them the following year.

To prevent outcrossing, tomatoes with an extruded style should be planted 75 feet from another variety. Tomatoes should be transplanted from the greenhouse to outdoors after being hardened off at temperatures no less than 55°F. Any plants with disease should be discarded. For uniform varieties, very few plants are needed to maintain the variety (as few as 5). For landraces with a lot of genetic variability, at least 20 plants should be grown to maintain the variety.

Some tomato varieties are parthenocarpic, meaning they have the ability to set fruit without pollination and fertilization. While this is an extremely valuable trait for producing tomatoes in cooler climates, it can present a frustrating challenge for seed producers. Generally the earliest fruits will have little to no seeds, and as the season progresses and both day and nighttime temperatures rise, pollination and fertilization will be increasingly successful and seeds will begin to appear in the fruits. If a variety is known to be parthenocarpic, planting a bit later than normal, so that fruit is developing when temperatures are consistently warm, may aid seed production. If a variety’s parthenocarpy is not known and the earliest fruits have little to no seeds in them, the variety is likely parthenocarpic and will produce seeds in later maturing fruits.

Before seed harvest, the tomatoes should be “dead ripe” but not over-ripe to the point of rotting. The seed should be extracted as soon as possible after harvest. Mash the fruits in appropriate sized containers — anywhere from small (pint) jars to 5 gallon buckets. Add water only if pulp mash it too thick to stir. Ferment the pulp mash for 2-3 days at 70-85°F.

Figure 2. Fermenting tomato seeds.

After fermentation is complete, add water and stir to help separate the pulp and seeds, pour off the pulp and light seed (decanting). Repeat decanting until only clean seed is left and all pulp has been poured off. Rinse clean seed through a strainer and spread out on a screen for drying.

Figure 3. Tomato seed spread on a screen for drying.

Seed dried to 5.5% or less moisture can be stored for 3-7 years under favorable conditions of 40-60°F and 40%RH or less.

Selection and Variety Improvement

Since tomatoes are primarily self-pollinating, selection for most characteristics should occur after the fruit has matured. Leaf shape and plant stature traits can be selected prior to fruiting.

Trait Timing
Soil emergence After planting
Seedling vigor After planting
Growth habit At flowering
Maturity Market harvest
Uniformity Market harvest
Vine cover Before flowering
Fruit type Before fruit maturity
External fruit color at maturity Market harvest
Internal fruit color at maturity Market harvest
Fruit shape Market harvest
Fruit firmness Market harvest
Disease Throughout season
Stem attachment Market harvest
Stem scar Market harvest
Blossom end Market harvest
Fruit cracking Market harvest
Fruit ripening Market harvest
Fruit shoulders Market harvest
Fruit smoothness Market harvest
Fruit brix Market harvest
Fruit weight Market harvest
Fruit length Market harvest
Fruit width Market harvest
Fruit diameter Market harvest
Fruit # per plant Market harvest
Flavor Market harvest

Disease

Start with disease-free seed and/or reputable transplants. Avoid planting into fields with a history of seedborne diseases. Use crop rotations. Compost or plow under all plant residues. Clean field equipment thoroughly if it’s been used in fields suspected of disease. Avoid using tobacco products when handling seed or plants as humans can vector Tobacco Mosaic Virus. Plant early in the season to avoid later-season diseases such as late blight (unless resistance is a desired selection trait in which case plant to optimize disease pressure). Rogue out plants showing symptoms of disease. Only collect seed from healthy plants. Wash fruit before processing seed. Conduct phytosanitary field inspections and lab testing on seed lots if disease is suspected. Conduct hot water treatment on seed lots that are known to have disease. Diseases are classified by severity with a class of 1 being the most severe and 3 the least severe.

Disease Type Severity class Favorable conditions Control measures
Anthracnose (Colletotrichum spp.) Seedborne 3 Hot & humid
Alfalfa mosaic virus (AMV) Seedborne 2 Proximity to alfalfa fields Hot water (25 min. at 122°F)
Bacterial spot (Xanthomonas campestris) Seedborne 1 Cool & moist
Early blight (Alternaria solani) Seedborne 2 Hot & humid Hot water (25 min. at 122°F)
Bacterial canker (Clavibacter michiganensis) Seedborne 1 Cool & moist Hot water (25 min. at 122°F)
Stem rot/canker (Didymella lycopersici) Seedborne 3 Poor soil drainage Hot water (25 min. at 122°F)
Leaf mold (Fulvia fulva) Seedborne 3 High humidity (>85% RH), wide temperature range 50-90°F
Fusarium wilt (Fusarium oxysporum) Seedborne 2 Hot & humid Hot water (25 min. at 122°F)
Phytopthora diseases (Phytopthora capsici) Seedborne 3 Poorly drained, compacted soils Hot water (25 min. at 122°F)
Bacterial wilt (Pseudomonas solanacearum) Seedborne 3 Hot & humid
Bacterial speck (Pseudomonas syringae) Seedborne 1 Cool & moist
Syringae leaf spot (Pseudomonas syringae) Seedborne 2 Cool & moist Hot water (25 min. at 122°F)
Tobacco mosaic virus (TMV) Seedborne 2 Warm & moist
Tomato mosaic virus (ToMV) Seedborne 2 Warm & moist
Late blight Foliar
Septoria leaf spot Foliar
Cucumber mosaic virus Foliar
Curly top (Western yellow blight) Foliar
Root knot nematodes Insect
Blossom end rot Calcium deficiency
Cracking fruit Physiological
Catfacing Physiological
Leafroll Physiological
Poor fruit set Physiological

This resource was made possible thanks to the Montana Department of Agriculture’s Specialty Crop Block Grant Program. 

Crops: Tomatoes

Publication Categories: How-to Guides

Tags: Seed Production, Seed Quality

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