The various symptoms of abnormal leaf shape or other similar leaf disorders were classified into three main types illustrated by a few pictures below and described in detail in the following fact sheets:
 
- stunted, vegetation;
- abnormal leaf size and shape (curled, curved, small, serrated, filiform, blistered, with enations on the leaf surface) (figures 1 to 3);
- perforated, cut, shredded leaves (figure 4).

Figure 1	Figure 2	Figure 3	Figure 4

When such symptoms occur, there are many possible causes:

•       either parasitic

- soil fungi in the case of stunted plants (see the section Root and collar abnormalities and lesions). Fungi such as Peronospora hyoscyami  f.sp. tabacina (formerly Peronospora tabacina), mycorrhizal fungi cause stunting of tobacco ("tobacco stunt");
- viruses such as tomato spotted wilt virus (TSWV), cucumber mosaic virus (CMV), alfalfa mosaic virus (AMV), tobacco mosaic virus (TMV), potato virus Y (PVY),  tobacco ring spot virus (TRSV) and other viruses (TLCV, BCTV, TRV, TStV ...);
- the phytoplasma that cause Stolbur;
- other pests, such as a Ditylenchus dipsaci nematode, bugs or other defoliator pests 

•      or non-parasitic

- polyphylla (known as "Frenching")
- genetic abnormalities, nutritional disorders, and various chemical injuries or hail damage.
 

In this context diagnosis is often difficult. As a matter of fact, diseases causing abnormal leaf shape often cause similar symptoms, making their identification difficult. For better understanding you should go through all descriptions and photos presented in related fact sheets. Frequently these diseases cause also Abnormal leaf colour. You are advised to refer to the relevant section

Frequently when one is dealing with this kind of symptoms he/she has to make one or several assumptions about the cause of the disease, with the exception of some rare diseases that demonstrate clear characteristics.