Seeds can germinate. Is the lot ready?

Before releasing a lot, you need more than a germination percentage

Seeds can germinate well and still carry pathogens. Before releasing a lot into the nursery, ask both “How many seeds germinate?” and “Do we know enough about this lot's seed health?” The outcome you need goes beyond pathogen reduction: seedlings ready for the next stage.

A result you can act on must answer three questions together, against criteria agreed before the trial:

• Target pathogen: has it declined enough to meet the agreed criterion? Use a test suited to treated seeds
• Germination: does it still meet the lot's acceptance criteria? Use the same method and assessment time for every group
• Seedling quality: are seedlings ready for the next stage? Count them using a definition agreed before the trial

If every criterion passes, test consistency and fit with your operation. If any fails, investigate and adjust before scaling. This page is written for seed companies, seed health or QA teams and seed-treatment R&D, with nurseries and growers as the ultimate beneficiaries.

Low contamination still matters at lot scale

Some plant pathogens travel with seeds, on the surface or inside them, so moving seed can introduce pathogens into new growing areas. An important example is black rot of brassicas, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc). ^[1]

In field work by Schaad and colleagues, a plot starting with 0.03% infected plants developed a high incidence of black rot in 1976, while a similarly low starting incidence produced different results in another year. Low contamination levels deserve attention, but no single value predicts an outbreak in every field. ^[2]

The table below is a hypothetical example of how a small rate translates into seed numbers in a lot, using 280 seeds per gram as a calculation input (1 kg × 1,000 g × 280 seeds/g × rate).

Seed-borne pathogens can move through production at several points:

• Germinating with infection: seed-borne pathogens may be transmitted to seedlings during germination
• Spreading in nursery trays: overhead irrigation can splash pathogens onto neighbouring plants in warm, humid conditions
• Carried with seedlings: latently infected plants may appear normal and be transplanted with the rest
• Spreading in the field: rain splash, irrigation droplets, tools and contact can support further spread

Xcc enters leaves through hydathodes at the margins and moves through the xylem. Familiar symptoms are yellow V-shaped lesions pointing toward the leaf centre and darkened veins. ^[1] Reducing the seed-associated source of infection is therefore one part of disease prevention, alongside water management, hygiene and control of other infection sources in the growing area.

Similar germination, different seedlings

In an experiment with naturally contaminated Chinese kale seeds, Suwannarat and colleagues compared untreated seeds with dielectric barrier discharge (DBD) and streamer corona discharge (SCD) plasma. Germination in all three groups was 95–96%, with no significant difference, but the proportion of healthy seedlings differed clearly. ^[3]

Germination alone would miss this part of the outcome, so the question shifts from “Will it germinate?” to “How well does this lot produce seedlings ready for the next stage?” Applying the 57% versus 78% difference to a hypothetical base of 10,000 seeds gives an arithmetic difference of about 2,100 seedlings.

“Healthy seedlings” must be read from each study's definition and assessment day. Standard seed tests use crop-specific criteria for normal seedlings, so the two terms are not automatically interchangeable. ^[4] The research proposes effects on both microorganisms and the seed surface, which may alter wettability and water uptake, along with exposure time, moisture and system design, but these Chinese kale results do not isolate each factor's contribution. ^{[3] [5]}

How cold plasma acts on seed-borne pathogens

Plasma is an energised gas containing charged particles and reactive species. Cold, or nonthermal, plasma is generated electrically: energetic electrons collide with oxygen, nitrogen and water vapour in air, producing reactive oxygen and nitrogen species (RONS) without heating seeds or water to extreme temperatures. Gas and sample temperatures still depend on the system and exposure time and should be measured during testing. ^{[6] [7]}

• Oxygen species such as O₃, •OH and H₂O₂ may contribute to oxidative damage to microbial components
• Nitrogen species such as NO, NO₂⁻ and NO₃⁻; NO₂⁻ and NO₃⁻ may occur in plasma-activated water depending on production conditions
• Ultraviolet light and charged particles can contribute to cell-surface effects in some systems

These species differ in lifetime and transport. •OH is short-lived, whereas H₂O₂ and NO₃⁻ may persist longer in water. Access to a pathogen therefore depends on the species, medium and pathogen location, and internal seed infection requires assessment beyond surface results. ^{[7] [8]}

The UC03 studies deliver reactive species to the sample in two ways ^{[5] [9] [7]}:

• Place the sample in the system while plasma is generated: direct and liquid-associated configurations such as ESDP (electrohydraulic streamer discharge plasma), which combines SDP (streamer discharge plasma) with activated water at the gas–liquid interface, and GLIP (gas–liquid interface plasma). Sample placement differs between studies
• Use water to carry reactive species to the sample: generate plasma-activated water (PAW) first, then expose the sample. Distinguish PAW activation time from its contact time with seeds or microorganisms

Before choosing a system, consider the steps your seeds must go through: can they contact water, will they need drying afterwards, and how many kilograms must be treated per cycle?

Alternaria: incidence declined as plasma exposure increased

Suwannarat and colleagues inoculated Chinese kale seeds with Alternaria brassicicola and compared how ESDP and SDP changed fungal detection after germination. Inoculation allows direct tracking of the target pathogen; a production lot also requires identifying its pathogens and contamination pattern. ^[5]

The clear trend is that incidence fell as plasma exposure increased, and ESDP produced a greater decline than SDP at the time points shown. This response can define an initial treatment range for a trial with an actual lot, but it still needs comparison with your current method in the same lot, monitoring of germination and seedling quality, and validation with naturally contaminated lots. ^[5]

Xcc: fewer infected seeds, but what about the seedlings?

Suwannarat and colleagues inoculated cabbage seeds with Xcc and compared GLIP with PAW. After treatment, the proportion of seeds testing positive for infection declined from 98% in the control to 37% with GLIP and 41% with PAW. ^[9]

The same study also reports liquid-phase experiments, counting colony forming units (CFU) per millilitre. These describe the pathogen's response in liquid, a different level of measurement from the seed results. ^[9]

The study also assessed healthy seedlings on day 10. The PAW group had a higher proportion of healthy seedlings than the untreated group in the reported statistical comparison. GLIP lay between them and was labelled “ab”, so it cannot be described as significantly different from either group. ^[9]

Compare with your current method in the same lot

Seed producers already have several treatment options. Placing plasma alongside your current method shows which approach fits your pathogen, seed and production workflow.

Find a range that reduces pathogens while seeds still pass

The goal is not the strongest condition, but a range that meets the pathogen target while seeds still meet your criteria.

Los and colleagues studied both microorganisms and seed quality in wheat and barley. Germination remained similar after short plasma exposures but declined after 20 minutes of direct exposure in the studied system, so longer exposure is not always better. ^[11] “Cold” also does not mean the sample always stays at room temperature: system design and exposure time affect heat accumulation and seed moisture, so monitor them alongside germination. ^{[11] [6]}

Rotondo and colleagues found that PAW acted against both pathogenic microorganisms and beneficial bacteria, and studied its relationship with pH and water chemistry. If seeds receive a biological treatment, check whether the selected process affects microorganisms you want to retain. ^[7]

Testing treated seeds, and keeping food safety separate

The ISHI (International Seed Health Initiative) method for Xcc in treated Brassica seeds emphasises detecting internal infection and checking whether residues or other microorganisms interfere with pathogen recovery. Its suitability for plasma-treated seeds needs assessment. ^[8]

Reports should specify sampling, seed count, test method, detection limit and controls, and “not detected” must be interpreted within the method's capabilities. Air- and water-based systems can generate reactive species without adding a formulated chemical, but process-generated species such as H₂O₂ and nitrogen compounds, and changes in PAW pH, still need assessment under the intended conditions. Include post-treatment rinsing or drying in the trial plan. ^{[7] [8]}

Edible young plants, especially sprouts, are another context where seed-associated microorganisms matter. An investigation of the 2011 German outbreak of E. coli O104:H4 linked illness to sprout consumption. ^[12]

Questions to answer before scaling

• Can plasma replace our current treatment immediately? Compare it with your current method within the same lot first
• Does stronger pathogen reduction always improve germination? No. Germination and seedling quality need separate measurements, and more intensive conditions can reduce germination, as the cereal study illustrates ^[11]
• Can plasma reach pathogens inside seeds? Pathogens beneath the coat or inside a seed may be less accessible than surface pathogens; verify outcomes with an appropriate detection method ^{[9] [8]}
• How long can treated seeds be stored? This article does not establish a storage life. Test moisture, storage conditions, germination and possible recontamination over the intended storage period
• When should we scale? When results are repeatable across lots and meet the agreed criteria; then establish batch size, cycle time, drying, throughput and cost using the actual system