Quite recently I had a letter from a very noted gardener, in which he said Sweet Peas were going out of cultivation to a great extent in his part of the country, because so many growers had their plants destroyed by streak disease. He added that it was his conviction that the disease was chiefly caused by over-manuring. This statement brings to mind a discussion at one of the Conferences of the National Sweet Pea Society on this same subject. Mr. Andrew Ireland, Messrs. Dobbie & Co.'s well-known Sweet Pea grower, gave his experience in this wise. He was asked to go and inspect a lot of Sweet Peas which had streak trouble. Like the practical man he is, he got a spade and turned up some of the soil in which the plants were growing. After doing so, he turned to the grower and said, "My man, it is not a gardener you should have sent for, but a sanitary inspector!" Against this, we have to put a statement made at the same Conference by Mr. Alexander Malcolm of Duns. Mr. Malcolm said that he knew of a lot of Sweet Peas in East Lothian which had gone off with streak disease, although planted out in perfectly fresh ground which had not been manured at all. Now I think it is just possible the cause of the trouble in both these instances was the same - the lack of the work of nodule-forming bacteria. In the first instance because the plants were overdone with nitrogenous manure; in the second because the bacteria were absent. My readers may not all be aware that plants of the order Leguminosce, to which the Sweet Pea belongs, are able to absorb nitrogen prepared for them from the atmosphere by the bacteria contained in the nodules on the roots of the plants. These nodules are easily observed by the naked eye. It is thus that a crop of peas or clover is a good preparation for a crop of a different character, because the ground after them is left richer in nitrogen.
Nothing is accurately known about streak disease, and no cure has been found for it. I have grown Sweet Peas in large and small quantities for twenty years at least, and I have never had any trouble with streak. I have seen a few plants here and there in a big plantation become sickly. Possibly they had streak disease, but we never took time to consider that - out they came and went the most direct road to the boiler furnace. The National Sweet Pea Society appointed a committee to investigate "Streak," and it also offered valuable prizes for a remedy, but nothing has come of either move. If I had a trench or a bed of Sweet Peas go off badly with streak or any other similar trouble, I should clear the lot off and burn them Then I should dig the trench or bed at once, fifteen to eighteen inches deep, and work in a lot of freshly slaked lime and leave it at that till early next spring, when I should dig again, and in April plant out my Sweet Peas as usual. Not a drop of fresh manure would I give if it had been heavily manured for the crop which went off. I should conclude that the manuring had been overdone and what the land required was sweetening and resting. After the plants got to their flowering stage, I would then feed them with liquid manure.
Other leaf and stem diseases are mildew and spot disease - allied to mould or blight. These are seldom if ever seen in well-grown plants. If observed in the early stages they can be successfully combated on lines similar to those followed when attacking mildew in roses, i.e. - dusting with flowers of sulphur.
Insect troubles are not numerous. The most serious is green-fly. I once saw this pest attack a big lot of plants so seriously and rapidly as to destroy them. It is much more likely to occur in the south than in the north. If its beginnings are carefully watched for, it can be stopped, and there is nothing better than the old-fashioned solution of soft-soap and quassia. Two to three ounces of soap thoroughly dissolved in a gallon of water and a tablespoonful or two of strong quassia extract, the whole well worked together with a syringe before applying, is a sure preventive of the fly spreading. If weather is wet, two applications may have to be made. The soap should be dissolved first in a quart of hot water and the rest of the gallon made up with cold water. If it is too troublesome or inconvenient to use soap and quassia, then I recommend a nicotine insecticide such as one of the "XL All" preparations used as directed.