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Safflower diseases

Safflower grows best with low atmospheric humidity and in deep soils with good internal drainage. Most of the diseases to which it is susceptible occur when soil conditions differ. Several diseases affect safflower in California. Some are widespread and others are more limited in occurrence. Crop rotation, careful irrigation practices, and planting treated and disease free seed are important methods for controlling losses from disease. Planting safflower where coastal fog is persistent in summer is not recommended because higher atmospheric humidity promotes Botrytis head rot, resulting in crop loss. Present commercial varieties have partial or complete resistance to Fusarium wilt and limited resistance to Phytophthora. The severity of Phytophthora root rot may vary depending on whether the crop is surface-irrigated, subirrigated, or not irrigated at all.

Phytophthora Root Rot. Phytophthora root rots (Phytophthora crytogea and P. drechsleri) are widespread and are the most serious diseases of irrigated safflower. Many races of the causal fungi are endemic in California soils and attack the roots and lower stem under favorable environmental conditions. No commercial cultivars are immune. Plants are susceptible at all stages of growth but the visible symptoms become more apparent from flowering onward. Infected plants become light colored, wilt and die. In the early states of infection roots may show a reddening of tissue. Infected roots and lower stems later become darkly discolored. Phytophthora can be devastating, particularly when crops are irrigated late in the season and plants have developed symptoms of moisture stress prior to irrigation. It may also occur in non-irrigated fields if heavy rains occur, especially late in the spring.

Infectious, motile spores of Phytophthora are released from fruiting bodies present in the soil when soils become saturated. The disease is most active when soils are completely saturated. A modest decline in soil moisture tensions reduces the activity of the pathogen substantially. Longer or repeated periods of soil saturation may be necessary for significant infection to take place on otherwise permeable soils, especially if initial inoculum levels are low and the cultivar has some resistance. Damage may be minor during earlier irrigations and then become widespread once a sufficient spore population has developed.

The resistance of safflower to infection by Phytophthora is not constant. The crop appears to be more susceptible to infection if drought stress has occurred, evident by firing or browning of the lower leaves. The extremes of over-irrigating and allowing drought stress to occur should be avoided. Although safflower apparently becomes more susceptible to Phytophthora the closer it gets to bloom, other factors may contribute to susceptibility. As safflower matures, temperatures also are rising and reach the range where the disease is most active. The likelihood that the crop has started to experience some water stress when the canopy has reached full size is greater and disease inoculum levels may be higher than earlier in the season.

The combined effects of different irrigation systems and soils with different drainage characteristics on the susceptibility of safflower to Phytophthora should be anticipated. On soils with poorer internal drainage and/or higher levels of inoculum, the risk of disease increases with succeeding irrigations. On shallow soils where pre-irrigation cannot supply sufficient moisture for a crop to mature, growing safflower based on frequent in-season irrigation is risky.

Good irrigation practice is made easier if pre-irrigation is applied or a full soil profile is available. Furrow irrigation can be efficient and is an inexpensive way to apply water to safflower, but soils may become more completely saturated compared to sprinklers. Sprinkler irrigation may allow water application at low enough rates to avoid excessive soil saturation. In turn, this will reduce inoculum buildup and shorten the period of time when soil near the base of the plant is saturated. However, sprinkler irrigation is expensive and is not available on all farms. For furrow irrigation, shorter and more frequent irrigation applications may be superior to less frequent, longer applications, especially if the soil has poor drainage characteristics. Only amounts of water sufficient to replace crop water use should be applied, whatever irrigation method is used.

Rust. Safflower rust (Puccinia carthami) is widespread in all areas of commercial production. The rust fungus has a complex life cycle and produces different types of spores, depending on the stage of the fungus' life cycle present. Black teliospores appear at the end of the crop growth cycle and can infest seeds or persist on the soil. These spores produce basidiospores, which then infect seedlings developing from infected seed or seedlings grown in a field that had safflower the previous year. Aeciospores and teliospores initiate the foliage stage of the disease on older plants and are wind borne. The foliar infection is characterized by chestnut-brown pustules (containing urediospores) on cotyledons, leaves, and bracts. These can re-infect the foliage in a number of recurring cycles.

Rust is more damaging when it occurs at the seedling stage than near crop maturity, when its occurrence usually has little affect on yield. Rust infection of seedlings can devastate safflower stands. Seedling infection occurs if seeds are contaminated with teliospores or if safflower has been grown in the field in the previous year. Infection at this stage results in girdling of the stem. Some plants die while others grow but fail to set seed. Girdled plants wilt from lack of water or can lodge in the wind.

A number of distinct physiological races of rust have been identified and others may well exist. Commercial varieties resistant to all races are not available although some varieties are resistant to some of the races. The use of rust-free seed is recommended for control, but even uncontaminated seed may become infected from seed cleaning and handling equipment. Seed treatment with recommended fungicides will give some, but not complete control of stand loss from seed-borne spores. Crop rotation, an essential practice, is effective in reducing stand loss from the soil-borne spores and prevents build up of spores in the soil from successive cropping. In general, late-planted and early spring-flooded fields have less rust infection than early-planted fields. Rust can be more severe on dense and heavily branched canopies.

Fusarium wilt. Fusarium wilt is found in Yolo, Solano, and Colusa counties and in the Sutter Basin. In other areas of northern California it has been reported in fields near the Sacramento River. This disease has the potential to spread and may become more widespread and serious. Characteristic symptoms are wilting and yellowing on one side of the plant beginning on the lower leaves. Older plants may be killed or lateral branches may die on the affected side of the plant. Young plants usually are killed. A brown discoloration of the vascular tissue occurs in the stem and roots.

The causal fungus is Fusarium oxysporum f. sp. carthami. It invades the roots and spreads systemically into the stem, branches, and leaves through the vascular tissue. The fungus persists in the soil and on plant debris, and is seed-borne in internal tissue of the hull and seed coat. Some varieties have partial and a few varieties are thought to have complete resistance to all the known races of fusarium. Growers should not save seed harvested from diseased fields for reuse, since the fungus can be disseminated to disease-free areas. Once fields are infected, there is a high likelihood of reinfection of subsequent crops. If fusarium infection is a possibility, earlier planting dates may help avoid serious infection.

Verticillium wilt. This potentially serious disease occurs in most safflower producing areas of the state. Plants are attacked at any stage of growth during cool weather but the disease does not rapidly kill the plants. Affected plants generally mature and dry up earlier than normal. A characteristic early symptom is interveinal and marginal chlorosis of the lower leaves. The leaves of the entire plant then progressively become mottled in appearance from the lower leaves upward. A dark discoloration may be evident in the vascular tissue of infected plants.

Verticillium dahliae, the causal fungus, is soil- and seed-borne. It is widely distributed in California soils, infecting cotton, tomato, melons, strawberries, and a number of other crops. The organism is favored by cool temperatures and fine-textured soils, high in nitrogen and moisture. The fungus invades the roots and spreads into the stem and leaves through the vascular tissue. All current commercial varieties are susceptible. Seed from diseased fields should not be replanted. Crop rotation with non-susceptible crops such as small grains, corn, rice, sorghum, and sugar beets is recommended.

Botrytis head rot. Botrytis head blight is a severe disease in coastal areas subject to continuously high atmospheric moisture. Infected seed heads become light green followed by complete bleaching. Whether seed is lacking, light in weight, or fully developed depends on the state of seed development at the time the seed head is infected. The entire infected seed head can be readily detached from its supporting stem because the bract area tissue is damaged.

Spores of the fungus Botrytis cinerea are wind-borne and widely distributed. Infection of seed heads may occur under favorable environmental conditions at any time from emergence of blossoms to seed-head maturity. No current commercial variety is resistant. Safflower should not be produced in areas where fog persists because this favors severe disease development.

Other diseases occur occasionally on safflower and are listed in Table 7.

Table 7. Diseases of safflower
Disease	Causal Organism	Damage	Control*
Alternaria leaf spot	Alternaria carthami; A. alternata	Brown, irregularly shaped spots on leaves and floral bracts; entire plants may become darkened; partial to complete crop loss	Rare in California but common in the Plains States and Mexico; partial tolerance in some cultivars; no resistant cultivars; no effective fungicides
Bacterial blight	Pseudomonas syringae	Brownish necrotic spots and pale leaf margins; lesions on stems and petioles; as disease progresses, rotted flower heads	Rare in California but common in the Plains States, Mexico, and other safflower-growing regions with higher atmospheric humidity
Botrytis head rot	Botrytis cinerea	Seed heads become infected, rot, and may fall off	Do not plant safflower in areas with persistent coastal fog in summer
Fusarium wilt	Fusarium oxysporum f.sp. carthami	Infects roots and spreads to branches; yellowing, wilt, and death; vascular tissue discolored	Crop rotation; use partially resistant cultivars; if disease is suspected, plant when soil temperatures are cooler
Phytophthora root rot	Phytophthora dreschleri; P. cryptogea	Infected plants wilt, become light colored, and die; vascular tissue in roots and stems becomes discolored	Irrigate before moisture stress occurs (do not saturate soil near base of plant); seed treatment (partial control); crop rotation
Pythium	Pythium spp.	Several species of soil fungi attack seedlings; preemergence or postemergence damping off of seedlings; usually in waterlogged or wet portions of the field	Avoid waterlogged conditions in planted fields
Rust	Puccinia carthami; several races	Seedling girdling and death	Rotate safflower with other crops to avoid seedling infection; use treated seed
Sclerotinia stem rot	Sclerotinia sclerotiorum	Infected plants turn yellow, wilt, and die; sclerotia are formed at the base of the seed head or in the stem; heads may fall off as the disease progresses	Very rare in California, but occurs in other safflower-growing regions with higher atmospheric humidity
Verticillium wilt	Verticillium dahliae	Early maturation and death prior to seed formation; interveinal chlorosis of lower leaves	Rotation with nonsusceptible crops like small grains, corn, rice, sorghum, and sugarbeets
* There are no economic thresholds for control of these diseases.

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Modified: 10 Feb 2000 Comments to webmaster@agric.ucdavis.edu