Diseases and developmental disorders of cucumber seedlings in greenhouses

Cucumber is affected by other viral diseases besides those mentioned above, but they are, at any rate, not of great importance for greenhouse crops. These include tomato black ring spot virus, arabis mosaic virus, and tobacco necrosis virus.

Non-pathogenic disorders

By symptoms, this disease is almost impossible to distinguish from true seedling damping-off or root rot, except for

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It is believed that such a disorder is the result of water stress shortly after seed germination, possibly due to the drying out of the compost or an excessively high content of soluble salts in the substrate, as in both cases, water uptake by seedlings is weakened.

Under favorable conditions, cucumber seed germination is completed three days after sowing. However, it sometimes occurs unevenly, and this can be caused by a number of reasons, of which the most common are the following.

Low temperatures. If the air temperature around seed trays or pots rises above 27 °С, germination is uniform, provided that all other factors are also favorable for this process. At a very low compost temperature, germination is greatly delayed and becomes uneven. In some cases, there may be noticeable differences in temperatures between individual germinators, adjacent pots, or sections of greenhouses.

Uneven moisture of the compost. In dry compost, germination is greatly delayed.

Insufficiently thorough mixing of fertilizers before their application. As a result, their concentration increases sharply in small areas of the substrate, and germination slows down.

If steamed soil is not used for more than three weeks, phytotoxic concentrations of ammonium or nitrite ions can accumulate in it. In such seed trays, germination proceeds normally along the edges but is severely delayed in the center (see pp. 88—89).

Pre-emergence damping-off, in particular, when affected by Pythium species.

Some of these signs may be of a genetic nature, which is cultivar-specific, but deformations much more often arise as a result of delayed seed germination or the presence of an excessively dense seed coat (old seeds). Deformed plants are best removed. Dissection of true leaves

Both in the nursery and after transplanting seedlings into the soil, the youngest leaves sometimes appear to be strongly dissected into a series of very unequal lobes. As a rule, only a small number of leaves are affected, and the plant develops normally.

The cause of such dissection is unknown, but their appearance is often preceded by a period of severe drought.

Cucumber is very sensitive to herbicides of the growth regulator type, but these chemicals affect the development of this crop to a much lesser extent than tomatoes. Symptoms of damage by herbicides such as 2,4-D, MCPA, 2,3,6-TBA, and picloram are very similar. Damage can occur as a result of drift during aerial treatment of fields adjacent to the greenhouse or if there are herbicide residues in the compost. This applies in particular to the slowly decomposing 2,3,6-TBA and picloram.

The first signs of deformation appear within a day of the plant beginning to absorb the herbicide and are expressed in leaf epinasty, which gives the plants a wilting appearance, although they do not lose turgor. At sublethal doses of herbicides, the epinasty weakens after a few days, but the further development of leaves is severely suppressed. The size of the leaf blades is significantly reduced, and as the plant recovers, they become strongly deformed, in particular the interveinal tissues. As a result, the leaves become blistered or cup-shaped, and sometimes turn over. Soil contamination with 2,3,6-TBA or picloram can be long-lasting, and the plant growth cultivated in this soil is impaired for several years.

In case of substrate (soil, peat, etc.) contamination with very persistent herbicides, it is advisable to replace the entire material, if possible.

On some cultivars, fruits with yellow streaks sometimes form, and very rarely, completely yellowed fruits. Such symptoms indicate genetic disorders, i.e., these are chimera plants.

If two or more shoots grow together, it is called the phenomenon of fasciation. This phenomenon is also observed in cucumbers. It is possible that it is associated with changes occurring at the cellular level in the growth point, which, however, cannot cause complete destruction of the meristematic tissue.

At the beginning of the growing season, the upper shoots of some plants often become stunted, take on a gray-green color, and necrosis develops along the edges of the leaves. In this case, it is said that the tops of the plants are burning.

As a rule, this is the result of excessive water loss by the leaves of the transplants soon after planting in the soil, which occurs when roots are damaged or sunny weather sets in. Sometimes the tops wilt, although wilting is not always associated with scorching.

After some delay, the plant growth usually recovers and continues to develop normally, provided the initial root damage was not too severe. Fruit abortion

In some cases, during the early development phases of cucumber, the vegetative growth of the plants is very vigorous, and immediately following this phase, a large number of flowers begin to form at each stem node and on the lower lateral shoots. Only some of these flowers develop into mature fruits; the rest turn brown, shrivel, and are often colonized by pathogens, such as B. cinerea. This browning and wilting of small female flowers and newly set fruit is called abortion.

It can occur at any time during the season when an excessive number of flowers form on the plants. Sometimes all developing fruits are aborted, and in these conditions, it is necessary to thoroughly examine the plants, especially the root system. Often, the drying of fruits is associated with a loss of the plants' root system.

Cold drought or a general short-term decrease in temperature during the development of the crop leads to the formation of typical symptoms on the developing fruits — slight browning, corking, and surface streaking. Sometimes it is evenly distributed over the entire fruit; sometimes it develops only on one side, especially as a result of drought. Apparently, the causes of such changes can be low irrigation water temperature and some pesticides.

Anon. (1969) Manual of Cucumber Production. HMSO, London.

Anon. (1972) The Biological Control of Cucumber Pests, Growers Bulletin No. 1. Glasshouse Crops Research Institute, Littlehampton.

Cucumbers, Lettuce, Minor Vegetables, Mushrooms. Ministry of Agriculture, Northumberland.

Morgan, N. G. and Ledieu, M. S. (1979) Pest and Disease Control in Glasshouse Crops. British Crop Protection Council Publications, Croydon. CHAPTER 9. LETTUCE