Soluble salt injury on begonia

Which begonia has been overfertilized? No foliar symptoms here. The small plants turned out to have very high electrical conductivity (EC) compared to the larger ones due to poor mixing of slow-release fertilizer. In short, a soil test is almost always needed to confirm a soluble salts problem. High growing medium EC can injure or inhibit the growth of plants. Common causes of high EC include: excess fertilizer (too high level or too frequent application), subirrigation, root disease, or restricted drainage.

Fact Sheets
Basic Fertilizer Programs for Containerized Greenhouse Crops
pH and Fertility Review for Vegetatively-propagated Annuals
Root Diseases of Greenhouse Crops