THE BIOLOGY OF LETTUCE ROOT APHID

Lettuce root aphid, Pemphigus bursarius L., is a pest which sometimes causes considerable and widespread damage to crops of summer lettuce.
Its favourite primary host is Lombardy poplar. On hatching in spring, from eggs that have overwintered on this host, the fundatrices cause the development of hollow, flask-shaped galls on the leaf petioles. A fundatrix becomes enclosed within a gall, where it matures, and gives rise to between 100 and 250 young. These fundatrigeniae are all potential alatae and on developing wings leave the gall and migrate to lettuce and other secondary hosts; they reproduce rapidly.
Lettuce sown between mid-April and the end of May is most severely attacked, while lettuce sown in July escapes attack. The alternative secondary hosts belong to the Compositae, subfam. Liguliflorae, and although lettuce is preferred by the fundatrigeniae, Lapsana communis L. and Sonchus asper (L.) Hill often have big colonies of Pemphigus bursarius on their roots. The size of the mature apterae varies between hosts and is largest on lettuce and Sonchus asper.
All winged alienicolae are sexuparae and fly back to poplar. Thus alternative secondary hosts are not reservoirs for lettuce-infesting alatae.
The sexuparae begin appearing on lettuce from about the last week in August. They generally come up to the soil surface and settle around the 'collar' of the host plant, to mature and develop wings. Having flown back to the primary host, the sexuparae seek out crevices in the bark where they produce arostrate sexuales. The ovipara contains one egg, and after mating she crawls as far into the bark crevice as possible and oviposits.     

THE USE OF TETRACHLORONITROBENZENE ISOMERS ON LETTUCE

Field experiments were carried out from 1951 to 1955 on winter lettuce plants grown in boxes under glass and in the open, and on commercial crops in unheated glasshouses. Each of the three isomers of tetrachloronitrobenzene, applied as 5% dusts at 1/4 and 1/2 oz. per square yard of bed, gave significant protection against Botrytis disease, but the 2:3:4:5 isomer was inferior to the other two isomers and they all delayed the hearting of the crop. The 2:3:4:5 isomer at both rates of application and the 2:3:5:6 and 2:3:4:6 isomers at 1/2 oz./sq.yd. delayed hearting of a commercial crop by more than 14 days and are not recommended at these rates. Local overdoses of fungicide should be avoided.     

EXPERIMENTS ON THE CONTROL OF BOTRYTIS DISEASE OF LETTUCE

Experiments were made during 1955-57 on the fungicidal control of Botrytis attack on lettuce, cv. May Queen, matured in frames from November to April, and cv. Trocadero Improved, grown as seedlings in frames from November to March and then matured in the field from March to June. The frame lettuce were treated during their whole period of growth and the field lettuce in the seedling stage only.
On both types, 0.33% thiram sprays applied on up to five occasions gave the best control of disease: 5% tecnazene dust was effective on the frame crop, in 2 out of 3 years when applied to the plants and also when incorporated in the top soil at the rate of 15 g. per sq.yd. Tecnazene applications did not give a good control of the disease on the field crop. Sprays with 0.25% captan were almost as effective as the thiram treatments on the field lettuce but not on the frame crop.     

THE ROLE OF THE SEED-COATS IN GERMINATION OF PHOTOSENSITIVE LETTUCE SEEDS

1. In order to analyze the role of both the fruit-coat and theendosperm in the germination of Grand Rapids lettuce seed andin the elongation of the embryo, the seeds were subjected toa number of operations. The fruit-coat was found to make onlya minor con tribution to the photosensitivity of the seed. Thefact that 100% germination of these seeds can occur in completedarkness, when the restraining influence of the endosperm isremoved by appropriate cuts, makes clear that it is the integrityof the endosperm layer that lecides the photosensitivity ofthe seed to red and far-red light. Thus it is deduced that theeffect of light is exerted primarily on the mechanical propertiesof the endosperm layer.
2. Neither red nor far-red light wasfound to affect the elongationof the radicle of de-coated seeds,whereas both powerfully affectthe germination of intact seeds.It follows that radicle elongationis not itself the limitingfactor in germination, although ofcourse it follows immediatelyupon the germination process proper.
3. In order to account forthe ability of red light to initiategermination, it is proposedthat the final step in the germinationcontrol process is theproduction of an enzyme whose actionenables the tip of theradicle to penetrate through the coat.This makes it possiblefor the radicle to begin elongating.Although the detectionof such an enzyme in the radicle wasnot successful, the cotyledonsdo produce both a pectinase andan enzyme hydrolyzing carboxymethylcellulose.Furthermore, seedsinjected with a cellulolytic, hemicellulolyticor pectolyticenzyme germinated to nearly the maximum extentin the dark.This gives considerable support to the hypothesis.

¹ Present address: Johnson Foundation for Medical Physics, Universityof Pennsylvania, Philadelphia, Pa., U. S. A. (Received December 5, 1962; )