Troubleshooting
'San Marzano' Tomato problems — and how to fix them
'San Marzano' Tomato (Solanum lycopersicum 'San Marzano') is generally forgiving once you match its basics, but a few issues come up again and again. Here is what each one looks like, why it happens, and the fix.
Blossom-end rot
Common on long paste fruit; dark sunken ends result from uneven moisture limiting calcium uptake. Water consistently and mulch rather than over-applying calcium.
Late blight
Devastating in cool, wet weather, causing brown leaf and fruit lesions; choose airy sites, water at the base, rotate crops, and remove infected plants promptly.
Lower yield from heat stress
Flowers may drop above about 32°C; provide steady water and some afternoon shade during extreme heat to hold fruit set.
Tomato hornworm
Large caterpillars defoliate plants quickly; hand-pick or apply Bt, inspecting undersides of leaves regularly.
Prevent 'san marzano' tomato problems before they start
Most 'san marzano' tomato issues are care-mismatch, not bad luck. Dial in the basics:
- Common tomato problems — the species this cultivar belongs to
- What's wrong with my plant? — the diagnosis apps compared
- 'San Marzano' Tomato watering schedule
- 'San Marzano' Tomato light requirements
- Best soil mix for 'san marzano' tomato
- 'San Marzano' Tomato temperature & humidity
- Full 'san marzano' tomato care guide
Looking for 'san marzano' tomato in general rather than its problems? Read the full 'san marzano' tomato care guide — light, water, soil and feeding.
'San Marzano' Tomato problems — FAQ
Why is my 'san marzano' tomato blossom-end rot?
Common on long paste fruit; dark sunken ends result from uneven moisture limiting calcium uptake. Water consistently and mulch rather than over-applying calcium.
Why is my 'san marzano' tomato late blight?
Devastating in cool, wet weather, causing brown leaf and fruit lesions; choose airy sites, water at the base, rotate crops, and remove infected plants promptly.
Why is my 'san marzano' tomato lower yield from heat stress?
Flowers may drop above about 32°C; provide steady water and some afternoon shade during extreme heat to hold fruit set.
Why is my 'san marzano' tomato tomato hornworm?
Large caterpillars defoliate plants quickly; hand-pick or apply Bt, inspecting undersides of leaves regularly.