Apples and Strawberries
It's the time of year to get your sprayers out of the shed and take them for a test drive. Green tip has already occurred in far-southern Iowa as this is written (March 25), and strawberries won't be far behind. After the rain-soaked nightmare of 1993, we're looking for a better growing season in 1994. To help insure a good crop, now is a good time to anticipate the major disease problems, take preventive action when possible, and plan spray programs when necessary. A look at the major springtime diseases on each crop:
Scab. This fungal disease ran wild in many orchards in 1993 due to frequent rains (18 rainy days in May!). Like many fungi, scab loves rain; wet periods lead to infected leaves and fruit. Because scab was such a problem last year, many orchards are carrying over large amounts of the fungus on fallen leaves and even on leaf buds. If you had significant scab last year, a rigorous fungicide-spray program is essential. Start spraying at green tip and continue at regular intervals (usually 7 to 10 days) through at least first cover. You can use the longer interval between sprays (i.e., 10 days) if you tank mix a contact fungicide (e.g., Dithane, Polyram, Captan, etc.) with an eradicant material (e.g., Nova, Rubigan) to maximize the combination of curative and protectant ability. If scab symptoms appear in 1994, a program of fungicide sprays (e.g., Captan) should be continued through the summer months. Consult Pm-1282, "Fruit Tree Spray Guide," from ISU Extension Distribution for the full range of chemical options.
Fire blight. Pruning out fire-blighted shoots and brach cankers is essential to tame fire blight this year. In addition, a fixed-copper spray around bud break can help to suppress multiplication of the fire blight bacteria between green tip and bloom. Cool weather during bloom has helped to suppress fire blight during the last two springs. However, a combination of warm weather and rainfall or heavy dew could cause damage on your susceptible varieties (Jonathan, Paula Red, Rome, MacIntosh, etc.). The highest-risk period for major outbreaks is the bloom period. If you have susceptible varieties of bearing age, and especially if the orchard has a history of any fire blight, you should consider applying streptomycin sprays during the bloom period. Some growers apply strep every 3-5 days during bloom, but growers in many parts of the world, including Iowa, have used strep more efficiently and effectively by timing their sprays with the MARYBLYT program. This program helps you to use daily temperature and rainfall information from your orchard to predict when the blossom-blight phase of fire blight will occur. You can purchase the MARYBLYT software from Pest Management Supply, Hadley, MA (1-800-272-7672), or participate in a cooperative MARYBLYT program with ISU (call me at 515-294-0579 for details).
Cedar-apple rust. This fungal disease is a serious problem on susceptible varieties in some parts of Iowa, especially where red cedar (the alternate host of the fungus) is numerous within a few miles of the orchard. The high-risk period is pink through second cover. As with scab, the most severe infestations occur when spring weather is persistently wet. Nova, Bayleton, Funginex, and Rubigan will control rust; Benlate, Topsin-M and Captan will not. If both scab and rust are problems for you, select your fungicides to protect against both diseases.
Leaf diseases. Leaf spot, scorch, and blight are foliar diseases caused by different fungi. Leaf damage is worst in wet springs on susceptible cultivars. Many newer cultivars, however, have fairly good resistance to these diseases. Scout your fields and apply a labeled fungicide (Captan, Thiram, etc.) if symptoms start to increase in the period before harvest. A good long-term alternative is to go with resistant cultivars when replanting. Consult Pm-1375, "Iowa Commercial Small Fruit and Grape Spray Guide," for additional spay options and information.
Gray mold. Also known as Botrytis fruit rot, this is often the number-one disease concern of strawberry growers. A key disease-avoidance strategy is to go easy on nitrogen fertilization in the spring, because a lush, overgrown foliage canopy sets up the moist, terrarium-like conditions that favor attack of the fruit by this fungus. The key period for fungicide sprays is bloom; a spray at 5% bloom and again at full bloom usually removes the need for any additional Botrytis-control sprays. Materials of choice for gray mold control are Ronilan, Rovral, Benlate, and Topsin-M. If the weather is dry, a single bloom-period spray is enough, but in a year like 1993, a third spray may be advisable.
Leather rot. Some growers may disagree, but I think this disease is unmatched in its destructive potential. You may lose berries to Botrytis, but you can readily lose entire fields to leather rot. The fungus that causes leather rot is active only under very wet conditions, especially where standing water occurs. The leather rot loss potential is greatest where soil drainage is poor. Even on heavier soils, however, you can usually keep the disease at bay with thorough mulching, which prevents splashing of disease-laden soil particles onto the berries. Ridomil and Aliette are labeled for leather rot control, and they can help, but if you have standing water and poor mulching, nothing short of divine intervention will stop leather rot. In other words: you've got to practice good cultural techniques to avoid leather rot if you're on heavier soils.
This article originally appeared in the March 30, 1994 issue, pp. , 1994 issue, pp. 36-37.