LEAF ROLL is quite distinct from the other virus groups. The downward rolling of the leaflets, its main symptom, is most pronounced in the basal part. In bad cases, opposite margins of leaflets may touch or overlap to form a tube. The leaf petioles are taller and spindlier and the leaflets smaller and narrower than normal. The leaf surface is ruffled and rugose. The color is pale green. Irregular yellowed areas vary in size. The leaf symptoms do not show in Fragaria vesca when grafts are made. All showed virus type 2 in the indicator perhaps that virus is a component of a leaf roll complex or merely was present in addition to leaf roll.

The disease is of little economic importance.

STRAWBERRY GROWERS are always searching for the best and most productive stock. Freedom from diseases, particularly virus diseases, is essential. Mostly, careful inspection of the plants only was used as a basis for determining freedom from virus. That is not enough: Virus elements might be in the stock without showing symptoms. Such stock, infected with other components of the disease, degenerates and yields less.

Indexing helps overcome the difficulty. The indexing usually is done in a greenhouse where all plants (but particularly the indicator) can be sprayed or fumigated and protected from outside contamination by vectors. The growth cycle of both the plants to be indexed and the indicators should be correlated to produce stolons of approximately the same age at time of grafting. The best unions are obtained when relatively young tissues are grafted. Hardened and unpliable stolon tissue seldom unites in a graft. Instances occur when the plant to be indexed fails to form stolons and a young petiole or flower stem must be substituted. This may be a less convenient graft but satisfactory if a union is obtained. In-arching, the grafting method most widely used, involves the cutting of tongues in opposite directions on the stolons being grafted and inserting the tongues together. The grafts are then bound with tape. The time required for transmission of the disease to the indicator depends largely on the time of year the graft is made. From late spring to fall, when light intensity, day length, and temperature are Most satisfactory, virus symptoms will show in F. vesca indicators in 2 or 3 weeks. Grafts made in the winter take up to 2 months for symptoms of virus to appear. Some components take longer to show in the indicators than others.

Indexing also can furnish evidence as to the amount of virus contamination in a given field, area, or section, or in any specific variety. It can reveal the types of viruses to be found in various parts of the country or the State. Indexing can be used to determine the presence of virus in wild strawberries growing near virus-free planting stock The wild stock, if contaminated, should be eliminated to reduce the hazard of reinfection in the planting stock.

ONCE A SOURCE of virus-free stock is found, it must be kept healthy. Different methods have been employed to fit local circumstances and varying situations. Isolation of the stock from the vicinity of other strawberries is essential. We know little about other hosts of the viruses, but very likely other agricultural crops harbor them. Further study may locate them. Because wild relatives of the strawberry might be reservoirs of virus, they have to be avoided when virus-free stocks are planted. The control of insect vectors is essential.

Some States have undertaken certification of stock to guarantee its purity and freedom from diseases, particularly virus troubles. Not all programs have been successful. Some failures occurred because of inability to verify the presence or absence of virus disease by careful inspection without recourse to indexing.

The California Department of Agriculture has initiated a system of certification in which indexing and isolation are employed. Planting stock of co-operating nurseries is isolated from general agriculture. Mother-plant beds are maintained under clonal separation. Plants of the progeny of each mother plant are indexed. The entire clone, if it is clean, is transferred to a mother-row propagation. Random indexing of that row is done, and if no virus is found the stock is released for field propagation. Certification is provided the following year for the stock that develops therefrom.

An experimental program for the production of virus-free strawberries has also been started in Oregon, Washington, and Idaho. Indexing is done to find virus-free plants to be used as base stock, which is grown under isolation or in screenhouses to produce foundation stock. The foundation stock is isolated and dusted to prevent reinfection with virus. From it comes the plants that are to be certified as being virus-free.

THE STRAWBERRY APHID, Capitophorus fragaefolii, carries strawberry yellows, witches'-broom, and crinkle. The aphid is widely distributed in the Pacific Coast States but is rare elsewhere in this country.

Capitophorus minor and an unnamed species of the same genus are found frequently in strawberry fields in the Eastern States. These two species of aphids have readily transmitted both of the eastern types of viruses to Fragaria vesca under controlled conditions. It seems likely that they spread the viruses in the field.

Five species of aphids Myzophis rosarum, Macrosiphum pelargonii, Myzus ornatus, Myzus solani, and Myzus porosus can transmit the viruses, but less readily than Capitophorus fragaefolii, the common strawberry aphid in the West.

Winged forms of the insects, capable of migrating considerable distances, are responsible for most of the spread. Very likely the wingless forms can Spread only a little virus from one Planting to another, but considerable migration of wingless aphids within a planting has been observed at a season when winged forms were absent. Winged forms of Capitophorus fragaefolii in the colder parts of the United States appear soon after strawberry blossoms begin to open and continue to be in evidence throughout the harvest season. Later in the summer, when temperatures are high, the number of aphids declines. This aphid can overwinter in its mature wingless state. The severity of winter weather has a bearing on the number of aphids that survive. In milder climates, as in California, winged forms appear in early fall and wingless aphids occur abundantly throughout the winter months. The population is low during the warm summer period. A species of Capitophorus, common in the East, has a similar life history to that of C. fragaefolii there, but its winged forms have been observed in the fall as well as spring.

Since aphids spread strawberry viruses, their control or suppression will reduce the incidence of disease. The best time to attempt to control the insects is when the winged forms appear. The control of aphids in a partly infected field will prevent the spread of virus throughout the field. New plantings can be kept free of virus if the aphid vectors are controlled. Parathion, tetraethyl pyrophosphate, and the gamma isomer of benzene hexachloride, applied as i-percent dust, are effective.

HAROLD E. THOMAS received a doctor's degree from the University of California in 1928. From that time until 1945 he was associated with the California Agricultural Experiment Station, where he worked on the diseases and breeding of small fruits. In 1945 he became associated with the Strawberry Institute of California, of which he is now director.

C. P. MARCUS, JR., has been a plant pathologist with the Department of Agriculture at Beltsville, Md., since 1950. His work has concerned the virus diseases and the production of virus free stocks of commercial varieties of strawberries.