Tomato fruits that develop hard center cores (white or green) are considered a disorder, not a disease. This disorder is most associated with white and woody interior fruit walls. When the outside walls are affected, this is referred to as "gray wall" and "Yellow Shoulders" affects the top or shoulders of the fruit. The good news for tomato growers is that these types of disorders can be avoided with proper care, fertilizing, and variety selection.
Factors leading to hard cores & other blotchy ripening issues
- The weather
- Fluctuating temperatures or specifically major swings in temperatures from cool to high, while fruits are developing, is a common trigger.
- Other weather-induced plant stresses can also contribute to hard tomato cores.
Just like with other common tomato disorders like leaf curl, sunscald, cracking, and poor fruit set, high temperatures, and plant stress can lead to hard tomato cores in some varieties.
- Nutrient imbalances
- Macro nutrients, as well as micronutrients must be in balance to get quality fruits.
- Improper levels of potassium (K) can adversely lead to hard tomato cores.
- Know what fertilizers and additives you are applying and why.
- Excesses of certain nutrients can limit the plant's ability to absorb other needed nutrients.
- Tomato plants need calcium (Ca), but high calcium interferes with magnesium (Mg) absorption.
- Excess calcium can also cause high pH, which leads to general micronutrient deficiency symptoms.
- Excess magnesium, in turn, interferes with the plant's ability to absorb calcium (Ca).
Soil nutrients and pH play a major role in how tomato fruits form and mature.
- Tomato variety
- Cultivars with juicy consistency tend to be more susceptible to hard cores, especially during adverse weather conditions or from nutrient imbalances as mentioned above.
Just like with cracking and blossom end rot, certain tomato cultivars are more prone to hardcore disorder than others.
Main Avoidance Measures
- Choose a proper variety for your region and growing conditions.
- Maintain proper soil nutrition, limit excess nitrogen, and be conservative with additions of micronutrients.
- Minimize plant stresses.
- Keep plants amply and consistently watered. Applying a minimum of 1 inch of water per week, per plant.
- Keep plants well mulched.
- Encourage good air circulation through and around plants by proper spacing and pruning when necessary.
- In high heat or areas with intense sunlight, shading plants can be helpful.