There could be several problems happening here. Many older crabapple varieties are more susceptible to diseases than newer hybrids. It's possibile that your tree has apple scab, a fungus that defoliates trees by midsummer. Consult a certified tree specialist for diagnosis, or take a sample of the leaves to your local cooperative extension office to confirm the presence of the disease. If it is apple scab, control the disease by applying a fungicide as soon as bud growth begins in spring. You'll need to repeat the application every 7-10 days for at least 5 to 8 times to regain control. Rake up and destroy fallen leaves. They contain spores that will reinfect the tree. If all this sounds like too much work, it may be time to replace your tree with a disease-resistant va-riety. Tea crabapple (Malus hupehensis) has good scab resistance, as do many named cultivars, such as 'Adams', 'Camzam' (Camelot), 'Cardinal', 'Donald Wyman', 'Guinzam' (Guinevere), 'Indian Summer', 'Prairifire', 'Robinson', and 'Sutyzam' (Sugar Tyme).