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Pest, Disease & Weed Timing CalendarsBacterial Disease Timing Calendar
Bacterial plant diseases are hard to treat once they take hold, so timing is mostly about prevention: clean seed, dry foliage and careful handling in wet weather.
Key points
- Bacterial diseases spread through water, wounds and infected seed, so risk follows weather and handling.
- Start with clean seed, rotation and weed control.
- Do not work wet fields, and keep foliage dry where possible.
- Get a lab diagnosis, since fungicides do not control bacteria.
How do bacterial diseases start and spread?
Plant pathogenic bacteria enter through natural openings such as pores in leaves and through wounds from wind, hail, insects, pruning and tools. They survive in infected seed, transplants, crop residue and sometimes in soil or on weeds. Splashing rain, overhead irrigation, wind-driven rain and handling wet plants spread them from plant to plant.
Because these routes depend on water and wounds, the calendar of risk follows rain events, storms and field work more than it follows a date.
For example, a hailstorm that tears leaves in a tomato planting creates thousands of entry points at once, so the days right after the storm are the time to scout closely and to avoid walking the rows while plants are wet.
Which weather conditions raise the risk?
Warm, wet and humid weather favors many bacterial diseases on vegetables and fruit, though some bacteria prefer cooler conditions. Storms with wind and hail create wounds and splash bacteria to new plants. After such an event, look for new leaf spots in the days that follow.
Learn which bacterial diseases are common for your crops from your extension service, since each has its own preferred temperature range and spread pattern.
Seed and transplant sources matter as much as weather, since a clean planting can stay clean through a wet season, while an infected one can start an outbreak in a dry one.
Where does prevention start?
Prevention starts before planting. Buy certified disease-free seed and transplants from reliable sources, inspect incoming plants, and keep infected material out of your fields. Rotate away from the host crop for the number of years your extension service recommends, and manage volunteer plants and weeds that can carry the bacteria.
For some crops, seed treatments and hot water treatments are described in extension publications, though they must be done as the procedure directs.
- Certified, disease-free seed and transplants
- Rotation away from the host crop
- Weed and volunteer control
- Clean tools, stakes and trays
How do you scout for bacterial symptoms?
Early symptoms often include small water-soaked spots on leaves that later turn brown or black, sometimes with a yellow halo, and spots on fruit that look raised or scabby. Some bacteria cause wilting, slimy stem rot or cankers on woody plants. Symptoms resemble fungal diseases and nutrient problems, so identification matters.
Take photos and send a sample, wrapped in a paper bag, to your state plant diagnostic lab through your extension office. The result guides what to do and what not to do, since fungicides do not control bacteria.
Keep notes on where symptoms first appear. Spots that begin near the end of a row, near a water source or along a path point to the route the bacteria took.
What field practices lower spread?
Avoid working in fields when foliage is wet, since moving through wet plants spreads bacteria. Use drip irrigation or water early in the day so leaves dry quickly. Space and prune plants to improve air flow. Disinfect pruning tools between plants and between blocks as extension methods describe.
Remove and dispose of infected plants early in small plantings, following your extension advice on whether to bag, bury or burn where local rules allow.
Wash and disinfect harvest bins and trays as extension methods describe, since bacteria can ride on equipment from one field or farm to another.
What can protective products do?
Some products are labeled to reduce the spread of certain bacterial diseases, and they work only as a protectant before infection, not as a cure. Their usefulness varies, and resistance can develop. Follow current local advice and the label, which is the law, including pre-harvest intervals, and protect pollinators. Applicator licensing varies by state.
Remember that sanitation and prevention do most of the work with bacterial disease.
Some growers ask transplant suppliers for their disease testing and sanitation practices, which is a reasonable question to put to any supplier.
What should your record show?
Write down the source and lot of seed and transplants, the dates of storms and wet spells, the first date of symptoms and the blocks affected. If a disease turns up, the record helps you trace its source, which matters for planning the next season and for talking with your seed supplier.
Review the record each winter and adjust the plan for the following year. For example, if disease showed up only in the block irrigated overhead, consider a change in irrigation for that block.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.