Do Rotations Matter within Disease Management Programs?

As agricultural land becomes more scarce, existing farms become more specialized, and land closest to roadside markets increases in value, in part due to the advertisement-value of growing some crops where they can be seen and/or harvested by customers, an increasing number of farmers are considering shorter rotations for some of their plantings. Indeed, it can be very tempting to shorten rotations. What would happen if one were to ignore rotations, that is, if one were to plant the same crop in the same field year after year? This was done in Penn State research fields for both tomatoes and for muskmelons. The results are summarized below in Table 1 (tomato early blight) and Table 2 (muskmelon/cantaloupe Alternaria blight).

Table 1. Defoliation associated with Alternaria early blight on tomatoes after growing tomatoes in the same field for 1, 2, 3, and 4 successive years).

Successive Years	% Defoliation when 5% Fruit were Ripe
Year 1	3
Year 2	30
Year 3	74
Year 4	70

Table 2. Date when Alternaria blight first appeared on muskmelons (cantaloupes) for successive years after muskmelons were grown repeatedly in the same field (from 1977 through 1981).

Successive Years	Years Grown	First Date when Alternaria Blight was first observed	# Days before August 8	# Days after June 1
Year 1	0	August 8	0	69
Year 2	1	August 3	5	64
Year 3	2	July 29	10	59
Year 4	3	July 25	14	55
Year 5	4	July 18	21	48

The results provide a clear indication of the value of rotations relative to diseases caused by pathogens that can survive either in soil or in association with refuse from diseased plants. Many vegetable diseases are in this category.
Traditional wisdom and common-sense, combined with results such as those presented in tables above, tell us that rotations are important. Interpretation of results from various field, greenhouse and lab studies, and observations by many plant pathologists, suggest a minimum number of years that a grower should avoid growing crops affected by specific diseases (See Table 3). All vegetable growers should consider this information seriously as they plan crop rotations within their disease management programs.
Table 3. Minimum years to avoid crops susceptible to specific diseases.

Vegetable	Disease	Period without a susceptible crop
Asparagus	Fusarium wilt & root rot	Indefinite; do not plant without fumigation
Beans	Root rots	3 years; use grain crops, including sweet corn in rotation
	White mold, Sclerotinia	3 years; avoid tomato, potato, lettuce,cabbage, celery, carrot
	Anthracnose	2 years
	Bacterial blight	2 years
Beets	Cercospora leaf spot	3 years
	Root rots	3 years; use grain crops, including sweet corn in rotation
Cabbage-related plants	Clubroot	7 years; avoid turnip, radish; adjust pH to 6.8 or above
	Fusarium yellows	Many years
	Blackleg	3-4 years; avoid turnip
	Black rot	2-3 years; avoid turnip
	White mold	3 years; use grains crops, including sweet corn in rotation
Carrots	Leaf blights (fungal & bacterial)	2-3 years
Celery	Leaf blights	2 years
Corn, sweet	Smut	2-3 years
	Yellow leaf blight	3 years
	Northern leaf blight	2 years
Cucumber	Scab, GSB, & leaf spots	2 years
Eggplant	Verticillium wilt	4-5 years; avoid tomato, potato, pepper, strawberry, brambles
	Fruit rots	3 years
Lettuce	Bottom rot (Rhizoctonia)	3 years
	Drop, Sclerotinia	3 years; avoid tomato, potato, beans, cabbage, celery, carrot
Muskmelon=cantaloupe &Watermelon	Leaf spots, GSB, & scab	2+ years; avoid other cucurbits
	Fusarium wilt	4+ years; watermelon Fus. wilt is different
	Gummy stem blight (GSB)	2 years; avoid muskmelon, pumpkin, squash
	Fusarium wilt	4+ years; muskmelon Fus. wilt is different
Onion	Leaf blights	1-2 years
Parsley	Damping-off	3 years
Parsnip	Leaf spot & root canker	1 to 2 years
Peas	Root rots	3 to 4 years
	Fusarium wilt	4 to 5 years
Peppers	Bacterial spot	2 years
	Phytophthora blight	3 years; avoid tomato, eggplant, cucurbits
Potato	Verticillium wilt	3-4 years without tomato, eggplant, pepper
	Sclerotinia stalk rot	4 years; avoid tomato, lettuce, beans, cabbage, celery, carrot
	Rhizoctonia canker	2-3 years; best with 2 yr. grass or 1 yr cereal
	Silver Scurf	2 years; primarily from seed tubers
	Early blight	2 years; avoid tomato
	Pythium leak; pink rot	4 years
	Common scab	2-3 years; no root crops; adjust pH to 5.2 or below
Pumpkin, & Winter squash	Angular leaf spot	1-2 years
	Black rot (GSB)	2+ years; avoid muskmelon, watermelon, and other cucurbits
	Fusarium crown and fruit rot	3 years; avoid other cucurbits
	Phytophthora blight	3 years; avoid tomato, pepper, eggplant, and other cucurbits
	Scab	2 years
Radish	Clubroot	7 years; avoid turnip, cabbage-related plants; adjust pH to 6.8
Turnip	Clubroot	7 years; avoid radish, cabbage-related plants; adjust pH to 6.8
Spinach	Downy mildew & white rust	2 years
Sweet potato	Black rot & scurf	3 years
	Pox (soil rot)	Few years; reduce soil pH below 5.2
Tomato	Bacterial canker	3+ years
	Bacterial spot	2 years; avoid pepper
	Bacterial speck	1 year
	Early blight	2 years; avoid potato
	Anthracnose	2-3 years; avoid potato
	Septoria leaf spot	1-2 years
	Fusarium wilt	3 years
	Verticillium wilt	Several years; longest possible; avoid potato and eggplant

Article adopted from Vegetable MD Online For original article