What if your tomato gets brown spots: identify late blight and septoria

TL;DR: Brown spots on tomatoes

Brown spots on tomato leaves are usually late blight (phytophthora) or septoria. Late blight strikes fast in wet periods and can destroy the whole plant. Septoria grows slower. Both you treat with pruning (remove lower leaves), better ventilation, and never spraying from above. In seriously wet summers: preventive copper spray.

What are those brown spots?

There are two main diseases:

Phytophthora infestans (late blight): This is the worst. It is the same fungus that attacks potatoes. You see large brown spots with water-soaked edges. Spots grow fast. Underside of leaf gets white sporulation. In wet periods your whole plant can die in days.

Septoria lycopersici: This grows slowly. You see small brown spots (3-10 mm) with grey centre and dark border. Concentric rings (rings) are typical. Plant gradually declines.

Difference: Late blight is acute, septoria is chronic. Late blight your plant in a wet summer can die completely. Septoria your plant dies from bottom to top, over months.

Both come from soil or seed. Both love moisture.

How do you spot these diseases?

Late blight signs:

• Large brown spots, irregular shape
• Water-soaked appearance (almost translucent)
• Underside of leaf: white sporulation or mist
• Growth is fast (day to day)
• Whole leaves can die
• Also stems and fruits can turn brown

Septoria signs:

• Small brown spots (3-10 mm)
• Grey/brown centre with dark border
• Concentric rings in the middle
• Growth is slow
• Leaves yellow gradually, fall off
• Stems usually not affected

Where do these diseases come from?

Late blight:

• Overwinters in soil and dead plant parts
• Spreads via water splash (rain, overhead spray)
• High humidity (>85% RH)
• Temperature 15-25 degrees Celsius
• Wet August periods are peak season

Septoria:

• Also in soil and on seed
• Spreads via water droplets
• Less humidity needed than late blight
• Grows slowly through whole season
• Can overwinter in compost

What should you do? Step-by-step

Step 1: Remove lower leaves

This is your FIRST defence line. Cut all leaves up to 30 cm above ground. These leaves are always hit first (because spores splash from soil). Repeat every two weeks, so your plant gets neater from below.

Step 2: Improve ventilation

Thin the plant. Remove side shoots and leaves touching each other. Ensure air passes through. This helps both diseases, as it dries leaves faster.

Step 3: Water base, not top

NEVER spray leaves. Always water from below (at roots), never from above. This is essential. Water on leaves helps diseases grow.

Step 4: Water morning, not evening

Water in evening, moisture stays on leaves all night. Water in morning, everything dries fast. This reduces disease pressure.

Step 5: Mulch around plant

Lay 5-10 cm compost or straw around plant base. This prevents soil spores splashing to lower leaves.

Step 6: Preventive spray at first signs

As soon as you see first spots (or in wet summers preventively by July): start spraying. Copper sulphate mix every 10-14 days. Spray underside of leaves well wet. Note: no spraying in full sun and above 25 degrees - leaf damage.

Frequently asked questions

Is my tomato quality still good if I prune early?

Yes! Lower leaves do not contribute much anyway. Besides: plant with fewer leaves is healthier and produces better. Fruit gets better because sun can reach it.

How long can I still save it if I see disease?

Depends on which disease. Septoria: still months. Your plant produces further, just prune and follow. Late blight: acute danger. As soon as you see large brown spots and white powder, act fast. Every day counts.

Can I still eat fruit if I already have some?

Yes, fruit is safe. Fungi sit in leaves, not fruits (unless very severe). Harvest fruit, throw away sick leaves. Fruit is good.

Does biological control help?

Biological prevention (good hygiene, ventilation, pruning) helps much. Biological sprays (Bacillus subtilis, etc.) help moderately. Copper sulphate (chemical but bio-allowed) helps better. In seriously wet summers: copper sulphate.

Frequently asked questions

Can I reuse compost with late blight?

Better not. Late blight sits in compost. Throw compost away if you had sick plant. Next year: fresh compost. This is worth the investment.

How long does late blight overwinter in the garden?

Years. In wet soil it can wait for years. This is why crop rotation helps (different place next year). But same ground next year: extra careful.

Are certain tomato varieties better?

Yes, disease-resistant varieties (resistant to late blight): "Phantasia", "Matina", "Eliza" are better. But none fully immune. Good culture prevents better than resistance.

Discover your own garden design

At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see where your tomatoes grow best - in spots with good air flow and sunlight. Prevention starts with good location.