Magnesium (Mg) deficiency is a plant disorder. Magnesium can be easily washed out of light soils in wet seasons. Excessive potassium fertiliser usage can cause also Mg to become unavailable to the growing plant.
Symptoms include, yellowing between leaf veins, which stay green, giving a marbled appearance. This begins with older leaves from late June, but spreads to younger growth. Can be confused with virus, or natural aging in the case of tomato plants. Fruits are small and woody.
Mg deficiency can be rectified in the short term by applying a foliar feed fortnightly, with Epsom Salts diluted at a rate of 200g per 10 litres of water (8oz per 2½ gal) after flowering. In the longer term add dolomitic limestone if soil pH allows, or other Mg containing rocks such as Kieserite. Reduce usage of potash fertilsers if this may be contributing to the problem.
Other physiological plant disorders include;