BREEDING FOR RESISTANCE TO CLADOSPORIUM FULVUM CKE. IN TOMATO

Leaf mold caused by the fungus Cladosporium fulvum has been a serious disease of glasshouse tomatoes all over the world. It is also important on the field crop where conditions of high humidity prevail. Breeding programs for resistance are under way in most countries. With the increased use of plastic in glasshouse construction, and the current interest in increasing CO₂ content, many tomato breeders believe that resistance to leaf mold will increase in importance. On the other hand, the trend to maintenance of glasshouse night-temperatures above that of the outside air on cold nights, and the use of air turbulators and automatic ventilation systems, have reduced the importance of this disease in some areas.

Leaf mold can be succesfully controlled either by the use of fungicides, by manipulating the environment, or by using resistant cultivars. Maneb is generally recommended for leaf mold control in Ontario. Zineb is used when the plants are small and in those areas where manganese toxicity may be a problem. (Use 80% maneb WP 2 lb. or 75% zineb WP 2 lb. per 100 imperial gallons of water and apply 7 ½ gal. to 1,000 sq.ft.) Relative humidity is probably the most important single factor in spore germination and growth. The relative humidity at the lower surface of the leaf must be 95% or higher. The optimum is 100% although a film of free moisture is not necessary. If, after infection, low relative humidity is maintained, chlorotic flecks will be formed where infection took place but there will be little or no sporulation. Back in the 1920's and early '30's, it was noticed that some cultivars would be less infected by leaf mold in certain locations than would other cultivars. A study of this phenomenon by Langford at our Station showed that there were two kinds or races of the leaf mold fungus - race 1 and race 2. These looked alike but some cultivars, notably Stirling Castle, gave a resistant reaction when attacked by race 1. This resistance is conditioned by the gene Cf₁. Race 1 could infect plants possessing Cf₁ but there was little or no spore production during long summer days. In the winter when light was poor, race 1 would sporulate freely on Stirling Castle. Race 2 would sporulate freely on all cultivars at all times of the year.

About the same time, workers in Germany found that the wild Red Currant tomato, Lycopersicon pimpinellifolium, was immune from leaf mold.

A number of people used it in breeding resistant cultivars. Alexander introduced Globelle in Ohio in 1938, Bailey, Langford, and Robb introduced Vetomold in Ontario in 1939, and Guba introduced Bay State in Massachusetts in 1942. All of these cultivars were immune from both races of leaf mold when they were introduced, but in about a year, at each of these locations, a new race (race 5) appeared to which these cultivars were completely susceptible. This Vetomold type of resistance, Cf₂, has been incorporated in many cultivars, for example, Single Cross, Tuckers' Forcing and Manalucie. It is still holding up in certain places where only a few races of C. fulvum are found.

After the downfall of Vetomold, the wild Red Currant tomato was still highly resistant to the three pathogenic races of leaf mold and in 1941 a cultivar having this resistance, Cf₃, was introduced in Ontario as V 121. V 121 was not immune from any of the races but was so highly resistant to all of them that the fungus did not produce any spores. Once again this type of resistance did not last long. In a couple of years, reports began to come in that V 121 was susceptible. Test inoculations showed that V 121 was still as resistant to races 1, 2 and 5 as it