Holes and tears in leaves can sometimes be caused by severe weather, but the usual culprit is a pest. Seedlings and lower leaves are attacked by the night feeders which hide during the day, slugs, snails, woodlice, vine weevils, etc. Above ground pests attack leaves growing at all levels included here are capsid bugs and caterpillars. Another group of pests do not make holes in the foliage but can be equally or even more debilitating. Examples include aphid and eelworm. There are two serious general diseases, powdery mildew which attacks in dry weather and grey mould which is destructive when it is wet. Yellowing between leaf veins is not a disease it is usually a sign of magnesium deficiency. Abnormally thin and twisted foliage may be a symptom of lawn weedkiller drift.
Many different leaf-eating caterpillars attack plants. Look for rolled leaves and large irregular holes in the foliage. Some are uncommon but a few such as the angle shades moth can be serious pests. Pick off the caterpillars if this is practical. If the damage is widespread spray with a persistent insecticide such as fenitrothion.
An abundant pest in shady town gardens, hiding under stones or leaves during the day and devouring young leaves of a wide range of flowering plants during the night. Woodlice favour plants which have already been damaged by a previous pest but fortunately this insect is rarely a serious problem. Control is not easy; do not leave rubbish in the garden.
Slugs and Snails
Serious pests, the result of an attack on young plants can be devastating. Irregular holes are formed and telltale slime trails can be seen. Damage is worst on a shady, poorly drained site. These pests generally hide under garden rubbish during the day, so keeping the area clean and cultivated is the first control measure. Scatter Slug Pellets around the plants.
The leaves develop brown areas between the veins; the plants may be killed if the infestation is severe. Aster, Calceolaria, Larkspur, Phlox and Zinnia as well as Chrysanthemum may be attacked. The closely related leaf blotch eelworm produces similar symptoms on Begonia. Pick off and burn affected leaves. Destroy severely infested plants.
Tiny black or black and yellow beetles attack seedlings of the Crucifer family (Stock, Wallflower, etc). Numerous, small round holes appear in the leaves.
Growth is slowed down and seedlings may be killed. The beetles jump when disturbed. Spray with Liquid Derris as soon as the first signs of damage are noticed.
The frothy white masses ('cuckoo spit') which occur on the stems of Phlox, Coreopsis, Campanula, Annual Chrysanthemum and many other plants are familiar to everyone.
Less well known is the cause, pinkish one-eighth inch froghoppers which suck the sap and distort young growth. Hose with water, spraying is not worthwhile.
Frost will severely damage or kill half-hardy annuals, and even a sudden cold but not a frosty snap in spring can affect developing leaves by destroying chlorophyll.
The affected leaf, when it expands, may be yellow-edged (Sweet Pea, etc) or almost white. Pick off badly affected leaves, feed to speed recovery.
The tell tale sign is the blackening and rotting of the base of the stem. The name depends on the plant affected, geranium blackleg, pansy sickness, campanula crown rot, etc.
Use sterile compost in seed boxes or pots. Avoid waterlogging. Destroy infected plants and water remainder with Dithane if there are still many plants to save.
The damping off fungi attack the roots and stem bases of seedlings. Shrinkage and rot occur at ground level and the plants topple over.
The golden rules are to sow thinly and never overwater. Ensure adequate ventilation under glass. Remove collapsed seedlings immediately if others nearby are to be saved. Water carefully.