Diseases are the important limiting factor for cotton production. In Thailand there were at least three potentially serious diseases of cotton - leaf roll, boll rot and bacterial blight. Leaf roll, the stunting disorder, most probably caused by a virus was the most important disease of cotton the country had ever had. The disease was widespread and extremely severe in most of cotton-growing areas. As a result, the growing areas had gradually decreased and only one-tenth left. Boll rot, a disease complex, was very serious in the past and bacterial blight was also observed to be severe. Minor diseases include preemergence and postemergence damping off of seedlings, and leaf spots caused by Colletotrichum gossypii (anthracnose) and Ramularia areola (mildew). There is the other leaf spot found causing extensive damage to the variety which was highly resistant to leaf roll last year. This variety was intended to be promoted as the next commercial variety. The unknown pathogen might be Helminthosporium gossypii or Alternaria gossypina since they were found on the diseased cotton plants. Wilts occurred sporadically and appeared to be of no practical importance in the commercial variety. The three potentially serious diseases said above have been satisfactorily controlled.

A varietal trial with nine imported cotton varieties including two local ones was conducted at the Bangladesh Agricultural Research Institute, Regional Station, Jessore in 1977-78. Seeds were sown on 21-8-77 and the harvesting was done in four installments from 19-12-77 to 10-3-78. The variety Coker-310 gave the highest yield 2025.20 kg seed cotton per hectare followed by Shanghai -204, D5/2. Deltapine-16, Curolina queen, DPy-1, MCU-5 and Khandopp while 2421-y gave the lowest yield 1559.11 kg seed cotton per hectare. The diseases found were anthracnose (Colletotrichum sp.), leaf spot (Cercospora gossypinal), leaf spot (Altemaria solani) and little leaf (Virus) but their occurrence and degree of incidence and the varietal variations were not notable. The important insect pests were spotted boll worm (Earias spp.), red cotton bug (Dysdercus cingnlatus), dusky cotton bug (Oxycarems spp.), leaf roller (Sylepta derogata), aphids (Aphis gossypii) and jassid (Kolla spp., Platyretus spp.). These were controlled by spraying carbicron.

The reaction on 14 varieties/lines of cotton (Gossypium sp) were studied against 17 different diseases in field condition under natural infection. Each variety/line showed different degrees of susceptibility and resistance to different diseases. The variety DPY/1 was found to suffer least degrees of susceptibility to various diseases than the rest. Kandwa/2 and 2421Y were also found promising in respect of disease resistance and disease avoidance. All the varieties/lines studied were highly susceptible (HS) to leaf spot disease caused by Alternaria sp and resistant (R) to foot rot and wilt disease.

Twelve different diseases (leaf spot, leaf spot complex, anthracnose, damping-off, foot rot, wilt, boll spot, boll rot, bacterial blight, nematode, mosaic and deficiency) of cotton (Gossypium sp) were recorded in a survey in 1974-78 at Dacca, Joydebpur and Ishurdi of Bangladesh. Among the diseases, bacterial blight, leaf spot, leaf spot complex, anthracnose and damping off were found to be as major and the rest as minor in status.