IPM Packages for Crops

diseases

Downy mildew, Mary Husbeck

DISEASES

Rust [ Puccinia porri (Pucciniales: Pucciniace- ae)] Small white flecks appear on leaves and stem which develop into orange colored circular oval shaped pustules. Heavi- ly infected leaves turn yellow and die premature. Bulb size and quality are also affected. Smut [ Urocystis colchici and U. cepulae (Uro- cystidales: Urocystidiaceae)] Symptoms appear as dark brown or black thickened lesions on cotyledons and young leaves. Large lesions may cause leaves to curve downward. Raised blister-like lesions may also occur near the base of scales of older plants. Infection progresses inward from leaf to leaf at the base of the plant. Infected plants become stunted and may die within 3-5 weeks after emergence. Iris yellow spot virus It is a tospovirus that is transmitted by onion thrips, Thrips tabaci. It causes straw-colored, dry spindle, or diamond shaped lesions on leaves and scapes. These lesions may have distinctive green centers. Dry, elongated lesions or flecks may resemble thrips injury. Infected seed stalks and leaves may lodge later in the growing season. Plant vigor and bulb size are reduced. Onion yellow dwarf virus It is transmitted from plant to plant by the green peach aphid, Myzus persicae, and other aphids in a non-persistent manner. Infected leaves show symptoms ranging from yellow streaks to complete yellow- ing. Leaves are sometimes crinkled and flattened and tend to bend over. Bulbs are usually solid but undersized.

Black mold [ Aspergillus niger (Eurotiales: Trichoco- maceae)] Post-harvest black discoloration at neck and clusters of black spores generally on or between scales. This disease causes the entire bulb to dry and shrivel. Bulb rot [ Fusarium oxysporum (Hypocreales: Nec- triaceae)] Yellowing, curling, and necrosis at the tip of leaf blades. Pinkish or reddish discol- oration appears at the basal plate of onion. When infected plants are pulled up, the bulb often detaches from the roots. Damping off [ Pythium spp. (Peronosporales: Pythiace- ae)] Infection usually occurs in seedlings and symptoms appear as water-soaked lesions on lower stems and a watery rot that oc- curs on the roots. Infected tissue becomes soft and constricted at the base, causing the plant to topple. Favored by ill-drained conditions or excessive moisture in field. Purple blotch [ Alternaria porri (Pleosporales: Pleospo- raceae)] A destructive foliar disease favored by cooler climate. Symptoms appear as chlorotic lesions on leaves with concentric black, velvety rings of fungal sporulation. These lesions then girdle leaves causing drying and falling off. White rot [ Sclerotium cepivorum (Helotiales: Sclero- tiniaceae)] Can be very damaging to onions. White rot causes pre-mature yellowing, wilting, and dropping of leaves, followed by destruc- tion of the root system and rotting of bulbs. Affected bulbs and roots are covered with a thick, white mycelial growth with numer- ous black or brown colored sclerotia.

Pink root

Downy mildew [ Peronospora destructor (Pleosporales: Pleosporaceae)] A serious foliar disease, common during cool weather. All plant parts during all growth stages are susceptible to this disease. Primary infection occurs when infected bulb is plated and secondary infection occurs when healthy plants are infected by spores produced by primary stage. Symptoms appear as brownish-pur- ple, velvet-like sporulation of the pathogen on healthy green leaves, more prominent in the mornings. Later, black mold develops on drying leaves. Infected leaves collapse and die in a short span of time if conditions are favorable. Pink root [ Phoma terrestris = Pyrenochaeta terres- tris (Pleosporales : Incertae sedis)] Infected roots at first turn light pink, then darken through deep pink to red and pur- ple. Roots become semi-transparent, wa- ter-soaked, and later shrivel and die. New roots may also become infected. Infected plants become stunted, bulbs shriveled and undersized. Severely infected plants appear to be nutrient deficient. The disease may cause death of plants when infected at the seedling stage.