Troubleshooting and late-season management

Blossom end rot — black, sunken patches on the fruit base — is caused by calcium deficiency triggered by irregular watering. It is not a disease. Once you see it, the affected fruit is lost, but consistent watering prevents it in future trusses. Blossom drop, where flowers fall without setting fruit, happens when night temperatures fall below 13°C or rise above 20°C. Wait for better weather; the plant will set again.

Late blight appears as dark brown patches on leaves and stems, usually in late summer during humid, cool spells. It spreads fast and kills the plant. Remove affected leaves immediately, increase air circulation, and stop overhead watering. If it takes hold, harvest all fruit and compost the plant. Paste varieties, with their dense foliage, are more susceptible than open-structured heritage slicers.

As September arrives, reduce watering slightly to concentrate sugars in ripening fruit. Fruit that is green by late September will not ripen outdoors — pick it and bring it inside. Wrap individual fruit in newspaper or place in a drawer with a ripe apple. They will ripen over 2–3 weeks. San Marzano and Roma ripen more reliably indoors than large beefsteaks like Brandywine, which need heat to finish well.